I've noticed certain foods give me diarrhea for a day or 2, then mucus & sometimes v.slight bright string of blood, then fine.what can i expect at dr?

I think there may be: a couple of explanations. You may have been exposed to a food bacteria experiencing only mild symptoms of food poisoning. OR your mucus/mildly bloody stool could be an allergic reaction to the food. Either case suggests a visit/evaluation by your Doctor. Expect stool cultures, possibly blood work. I would put my money on recurring food allergy/sensitivity which, hopefully, tests will identify.