Food cannot cause you to flare - it can only aggravate the symptoms. I'm in remission now and can eat pretty much what I want - with moderation. I just know - that for myself - if I eat too many fruits & vegies in one day - I will have extra bathroom trips the next day. That does not mean I am flaring - it just means I no longer tolerate that many fruits & vegies.
If you want that piece of pizza - have it. The only way you'll know if it's bad for you is if you try it first. If you have symptoms like cramping or excess diarhea afterwards - you'll know for next time to limit your pizza intake.