Cramping, Pelvic Pain, Ultrasound Normal. What Could Be Causing It?

My name is Michaela Cushinberry I have the iud mirena and have had it now for about two years my first one dislodged itself so I got another one inserted. Just about 3 months ago I started to have server cramping two weeks before my period as well as sharp pains about three or four times a week in the pelvic area and the anal area like there is pressure July 2012 I went to get and ultrasound done at the request of my OBGYN and they sound nothing things were normal. My question is what could be causing the knife like sharp pains in my pelvic and rectum area? Please help P.S I have no fever, vomitting once after a meal no more after that, no abnormal discharge, no smell, no itching, no pain durning intercourse, everything seems normal just the terrible pains.