During my period, i experience extreme pain in my lower butt. It is as in something is pulling itself down, and cause extreme pain as in knife poking. It happens all the time during my periods and often on 3-4 days, not necessarily the initial 3-4 days. It goes with a pain killer mostly, but sometimes doesn't affect that also. My periods are also very elongated upto 9-10 days sometimes. I read on the Internet that these are symptoms of endometriosis. Is that a possibility? How to be sure?