What Causes Alternate Bouts Of Diarrhea And Constipation?

For some time -- months-- I have suffered alternating bouts of constipation and diarreaha, both causing discomfort and seldom letting up. I've just made two trips to the ER over what looks like gall bladder. dMy lower intestines for the last few weeks have been unremitting painful, with almost no relief. My upper GI is to be tested with a text called HIDA for gall bladder, but I assume this doesn't deal with the lower intestine. It seems that an intervention needs to be done, but what? What doctor to approach. I am a white woman, 76 years old till now always in great health.