A nutritional therapist would be able to help you with this but tbh you might as well give it a go yourself.
Remove each food from your diet for a minimum of 2 weeks and see if you feel any change. If not, go onto the next one. Bear in mind that it could be more than one food that is upsetting you, so don't reintroduce eliminated foods at this point.
Start with wheat, a common offender. this means no biscuits, cake, bread, pasta - unless they come from the Free From section in the supermarkets.
Next - give gluten a miss - this includes rye, barley and possibly oats (although the gluten in oats is quite different - even some coeliacs are ok with it, but severe cases have to avoid it as well)
Next to go is dairy. Switch to soya or rice milk.
If after eliminating these groups, you still experience no change, it gets a bit trickier. Personally I find tomatoes do it for me - can't touch the buggers.
When you think you might have found a potential trigger, after 6 weeks without it, reintroduce it in a very small amount - your bod should react quite strongly to it (as it will have had a respite from it and be very unwilling for you to bring it back) if it is a true trigger.
It's a start anyway - HTH!