What Causes Soreness And Stabbing Pain In The Uterus?

I m 20 years old. For the past two months or so my uterus has been very swollen, hard, and sore. Like a stretching or throbbing pain. My breast have gotten heavy and sore. My nipples have gotten larger and have tan rings around them. I ve also felt very tired, sick, and light headed. I thought that I might be pregnant but all the tests came up negative even the pee test at the Emergency room came up negative. They tested me for infection and std s and they came back negative as well. They refused to do an ultrasound. And sent me home. I ve been having my period, I think. My last three periods have been very different than usual. They now only last 3 days instead of the usual 6-7. They have also been very very light and the blood is pink or brown, and there is no clotting anymore. I m so confused. Please help.