Is it Gall Bladder disease? Colitis? Or an Ulcer?

I went to the dr's yesterday for constant diarrhea, like 3 or 4 times a week.Constant pain in my left side and back and sometimes in the center of my stomach by my bone. He seems to think it is one of these 3 things and wants me to have a ultrasound... Which I have scheduled for Sept 8th. I don't go back to him until Sept 29th for my follow up unless something shows up on the ultrasound.In the meantime he gave me Zantac 300 to take twice a day... I would like to know what I am dealing with here. I am a lot scared and would like some advice ... Please help!