How to Communicate Relationship Worries

Hey all,

This if my first post here.

I'm dating a genderqueer person who used to be in primarily open relationships. They have said that they sometimes doubt the ability for monogamy to work out, but they also have said that they believe a relationship is closed unless otherwise stated - and I have said plainly that I do not ever want an open relationship. We have never had trust problems so far, but at the moment they are away for a week visiting friends in another city, basically partying it up with them. All of their friends are LGBT and many are anti-monogamous. All of a sudden I have been overcome with feelings of doubt, worry, and paranoia about faithfulness. I haven't been given any reason to feel this way except for over-thinking. The only thing that they have done to provoke it is not replying to texts often and saying they couldn't call because they're going to a house party.

I feel like a bit of a tool for feeling insecure like this, to be honest. But I'm aware that this will need to be worked out for the future because I hate feeling like this.

Does anyone have any advice on how I should go about bringing up my worries without sounding overbearing or clingy? Should I wait until they get back in a week? I really don't want to say the wrong thing and scare them off.

Thanks in advance.