What Causes Prolonged Periods And Tender Breasts With Negative HPT?

Hi Oct. 20, 2010 was the last time i had my period and it lasted a week and a half. every since about 3 in a half weeks ago my breast (both at the same time) has become tender off and on to the point i wanna take tylenol and i also been having cramps with no period, and the past 2 weeks my left lower tummy hurts too. I took a store brand pregnancy test and its negative, i hope you guys have an ideas what could be wrong.