What Causes Severe Stomach Pain During Bowel Movement?

Hello, I have been having horrible stomach pain, I believe its stomach, when I have a bowel movement. Not every bm. My BMS are normal color and I go 2-3 times a day. I have been to the ER twice. They did an ultra sound on my gallbladder, was normal, and the GI cocktails they have given me do nothing for the pain. The only relief I get is to get in the shower as hot as I can stand for 20 or 30 mins at a time but even that is not giving relief much anymore. I have been vomiting the last couple of times. The ER Dr says he doesn t understand why its only when I have a bm and again its not everyday. Please help. I do have an appointment w a specialist but in the meanwhile I am in agony.