What Causes Lower Abdominal Pain, Back Pain And Irregular Menstruation?

I have been experiencing sever lower abdominal pain for at least five months now. I recently started to have a period every two weeks. At one point i found blood in my stule. It hurts to have rough intercourse.i have had four pregnancy tests, negative. An altrasound, negative. And blood smaples taken, and all were negaive. I take birth conrol. Ortho Tricyclin low. Why am i having so much pain!? Its also in my lower back. This isnt a cramping feeling. This is a sharp intence feeling. Please help me.