What Causes Early Menstruation, Dizziness, Lower Back Pain And Tender Breasts?

Hi i was suppose to get my period March 3 i started bleeding February 27. This has never happened before. I had unprotected sex so there is a possibility i could be pregnant. I took a pregnancy test February 19 and it came out negative. The bleeding started out brown turned to light pink and now its dark red with some clots. I am having pregnancy like symptoms such as swollen/tender breast, lower back pain, and dizziness. The bleeding isn t heavy its like medium (not enough to fill a pad for hours.) I don t think it was a miscarriage because im not bleeding heavy. Please help!