Try to keep a food diary and write down everything and see if you see patterns.
I myself over the years, have had to completely stop eating many foods. I think the first thing was eggs, then onions, peanut, and milk. I also have had to stop red meat and basically anything you fry because of the grease and fat.
I also cannot have butter and tomato base things.
I pretty much eat a very regimented, same diet of fresh sandwhiches and Tyson nuggets. Yes it is boring, but I know what my consequences are.
I hope that you can maintain a variety and hopefully not have to eliminate a lot. Sometimes you can have things in moderation. I also try to avoid "small" pieces like corn and tater tots, the same thing happens to me!
Also, be aware of hidden foods. Like for instance, peanuts really bother me and you can find peanut residue in chocolate candy and then some people cook with peanut oil.
I read a lot of food labels and thank goodness, there is a new FDA law that labels have to be more simple and straightforward, like instead of casein, it will say MILK. And then I say RUN
Good luck and God Bless.