I was in Jamacia last year for week and had the same problem. I wasnt sure if it was a parasite or something I eat or all of the free booze. My symtoms were alot like your. Had to go and go NOW. There was no waiting around. Had to go right after I ate and first thing in the morning. I have problems with my stomach to begin with and this did not help. But when I started seeing blood on my stool, I was alittle worried.
If the doctors see nothing wrong yet chances are you are ok. Futher test will show anything wrong. I know you may be already going too much now, but a good bowel cleansing might help, if its not too much on your stomach.
Good luck ,