Your immune system may be "triggered" into a flare if you cannot tolerate a food or have a food allergy so everyone is different about that. I agree that you need to just keep your own notes about what foods you tolerate best. Ask your parents (or someone who cared for you growing up) about foods you typically had problems with.
If something gives you diarrhea or other problems then avoid it.
Gluten, the protein found in wheat, rye, and barley, is in most of the grain-based products, such as cereals, breads, and pasta. If you never had a problem with gluten before you started your flare then its unlikely gluten is something to be concerned with.
You may just want to avoid higher fat dairy. It’s really the fat that gives people problems during a flare. Some people are lactose intolerant but again, that’s something you would have noticed before you had your flare.
For most, foods will not cause a flare or get you out of one but low fiber diets will make a flare more comfortable when food is passing if your flare has considerable amount of ulcers well up the colon. Some, with more mild flares, actually add a little fiber to assist with their bowel movements.
I agree to avoid gas producing foods. Gas makes your colon more active, it smells and the gas can be uncomfortable.
Add a probiotic to your diet.