I do have digestive issues, mainly celiac disease. Have been gluten free for 4 years due to that. Before being diagnosed I had to give up milk because it just caused me a lot of gassy pain and discomfort. 2 years into being gluten free and I could handle diary products a hell of a lot better, something my doctor said would probably happen. But my relationship with milk has never seemed to recover. Sometimes I would drink it and be fine, other times I would drink and be in gassy pain. These last few months any time I drink a glass of milk it is straight up pain within 10 minutes, and then I feel like shit the rest of the day. Funny thing is is that I could of had a bowl of ice cream (I pour milk into my ice cream, I refuse to eat it any other way) like 2 or 3 hours before and be perfectly fine. Decide I want a small glass of milk= pain within 10 minutes, often gassy and often requiring a trip to the bathroom. Does anyone have any theories on to why this might be? And I don't know if it has anything to do with it, I only drink 2% and skimmed milk. Never been a huge fan of whole milk.
I guess some products are just harder on our bodies than others. I'm lactose intolerant and I've found that while drinking milk is hell, ice cream isn't half as bad. The same applies to other products (eg. biscuits are fine, pastries suck). I don't know if different people are affected by different foods, but I imagine you've just got to use trial and error to find the things you are ok with.
Research casein intolerance. This is the protein in dairy which many people are unknowingly sensitive to. I am the same exact way. I typically avoid dairy because it hurts my stomach and causes bloating and gas. Aside from this, too much makes my skin breakout. However I've noticed the stomach issues are only with dairy that has protein. I can eat all the butter and ice cream in the world and I feel fabulous, but Greek yogurt and milk has my gut knotted in pain. There have been anecdotal testimonies about the effectiveness of a GF-CF (gluten free, casein free) diet insofar as physical and mental health are concerned. It is commonly recommended for people with autism spectrum disorders for these reasons, and also because gut issues are more common with ASD. Worth looking into.