Help!

I'm 11 weeks PP and I had my first period at 7 weeks PP. I'm due to have my period this Friday and I'm currently having REALLY sharp pains in my uterus. It hurts when I stand and sit and if I sit a certain way its like a jolt of pain. I took a pregnancy test and it was negative. I never have had this type of pain before a period before. Is this normal? What could it be? How long should I "wait it out" before I go to my OBGYN? Should I also take another pregnancy test later in the week? I'm not on any birth control and only taking a pre natal vitamin and probiotic.