I Have Been Struggling With Diarrhea Type Issues For Months

I have been struggling with diarrhea type issues for months now and the doctors just tell me it's because I have anxiety, but I know in the pit of my stomach there is something more serious going on. I can't eat anything that has milk, gluten, high fructose corn syrup and some types of meats because they go right through me.
Yesterday I had a smoothie with a Pitaya in it, and this morning I woke up with bright red stool. I am not sure if it's blood or from the Pitaya. What do I do?