What am I to her? What do I do?

So this will probably a pretty long post, but I just have to get this off my chest, I need some advice and none of my friends or family really gets this - sorry for that and thank you to anyone willing to read this:

About a year ago a new girl starting working at my workplace (a delivery service) and at first we didn't really talk to each other (I was pretty shy back then and she was just trying to fit in). Just before Christmas we got closer, talked almost all of the time about everything, but since we were both busy with school / college we never really spend time outside work - which was fine because we saw each other almost everyday there.
We exchanged phone numbers, she added me on Facebook and began liking / commenting things like "I wish I was there with you!" on everything I posted, no matter how ridiculous it was. We also texted a lot, but it was only when I was celebrating Christmas at home with my family that I realized I wanted her with me (until then I had never fallen for a girl, so it took me a while to come to terms with my feelings for her).

I spent the next months trying to get to know her better, spent more time with her (convincing other co-workers under some pretence to swap shifts so I got to work with her more / alone with her etc.)
And the more time I spent with her, the more I learned about her, the more I fell in love with her. Not even just her looks (she is absolutely beautiful) or her confidence, everything about her. Her voice, her smile, her laugh, the way she sang along to the songs in the radio, the way she was singing children songs to cheer everyone up during a tough shift, basically how she is always able to make everyone's day better.

While this might at first have been not more than some crush (which at the time I didn't take very seriously) I soon enough found myself having dreams about her, thinking about her in college or at home, in vacation - everywhere.

Problem is: she has a boyfriend and as far as I know is straight (though I don't know for sure - she knows I'm bi and is very accepting, one of her best friends is also a lesbian). So I tried to move on, tried to forget about her (as hard as it is when seeing her three times a week).
But, here comes the hard part: once I had resolved to try to forget about her she basically started flirting with me; she began touching me more and more, standing very close to me (or even leaning all over to the other side of the table right in front of and leaning against me instead of just going to the other side), smiling whenever I looked at her and staring at me like all the time, trying to involve me in everything and one time even asked her best friend to visit her / us at work, so she could introduce us.
I first I thought I was just imagining things (we work in a very tight space and it's pretty normal to rub shoulders with someone), but she still manages to keep her distance to everyone else at work, even to people with whom she has become good friends.
At some point I even came so close to her that she asked me instead of her boyfriend or one of her best friends to go to the doctor with her, when she needed a cast ; that I was her first contact person when she had a car accident or lost her phone.

So a few weeks ago I decided to take things a small step further and asked if I could come over to her house one evening to see how she lives (she kept telling me how she has so many pets at home and it's a "mini-zoo") - she practically jumped at the chance, was smiling and told me we could bake together (we both love cooking / baking), watch a movie and take a walk with her dog, she even gave me her address - all we were waiting for was our duty roster for the next week to find a free evening (with college till 4 pm and work often till 11.30 pm this wasn't easy).
Then she never talked about it again and when I texted her and asked if Wednesday would be okay she didn't respond and texted me two days later "I didn't get your message until now" - when I asked again a week later she just ignored me.
After that I stopped texting and talking to her, I was just so mad at her...I guess she realized that since she didn't try to cheer me up this time, literally gave me space and even defended / shielded me when someone else was trying to talk to me or tell me to "cheer up".
A few days later things got back to normal, we were talking and laughing together all the time, standing real close to each other.
All this time I was just trying to laugh, smile, enjoy time with here and pretend like she is nothing more than a co-worker, a friend. Even when I had the opportunity to tell her otherwise (last week I dropped a bottle when someone at work asked if I was "lovesick or something" with her standing right there) I tried to play it off as "just being tired".
But the truth is: it's getting more and more difficult to work with her, while there was a time when I was just happy and looking forward to seeing her again, right now I'm just trying to avoid her as good as I can at work - and I still feel like crap.

Anyone who really took the time to read all this...thank you, you are amazing!
The thing is: I'm really desperate right now and I don't know what to do. I'm pretty sure she's straight (and even if she always complains about him - she has a boyfriend!), but then again I don't think I can do this much longer. I've always convinced myself that I have nothing to gain by telling her, but could lose her friendship or even - worst case scenario - her at all, get problems at work etc.
For the past few weeks that worked well, but right now I'm pretty much ignoring her, don't really want to go to work when she's there (which she is, most of the time) - I don't really know how much worse it could get.

So..should I tell her? If so, how? (we are never really alone at work and I'd hate to do this by text)? And what do you think of her behavior? Could be scared?

 

I kind of tried that back then,when she first ignored my question when a good time would be I asked if she was only joking when she basically invited me to her home or if something had happened in the meantime so that she no longer wants this.
Her only reply was that "of course she meant it" and that we just had to find a date the next week, since she would be going to the zoo the next day (the only day off we had that week) - only to ignore me again the next week when I asked again.

You are probably right, I should have asked again and again but that was weeks ago and I'm not sure if bringing this up again now would be a good idea.


Thank you for your replies!