Belly on Pain!!!?

For the last 8 months, I have been experiencing really bad belly on pain. It's sharp but almost like a burning. And it is constant. To touch it's excruciating. It starts every month usually a few days after I start my menstruation cycle, the pain gets worse as the week progresses, and last up to about a week & half AFTER I am finished menstruating. I have been to the hospital twice for this pain, and no doctor can tell me what it is, only what it isn't. It is not a bladder infection (or infection of the intestines), it is not an umbilical hernia, I am not pregnant, it is not appendicitis, or a urinary track infection etc. Nothing has been detected with urine samples or an ultra sound. Does anyone else have this problem? Or has been diagnosed with something to explain the pain??? (And no it is not menstrual cramps, I know the difference, I get that too, as well as the pain directly behind my belly on. Anyone else have this problem? Any suggestions?