What Causes Heaviness In The Abdomen And Sore Breasts?

Hi, my last period was June 6, 2010. I am having some heavy feeling not really hurting but more like pressure in my lower abdomen. Also my breasts are sore and heavy. I am not taking birth control as of right now. I did take a test and it was negative. Could I be pregnant, and if not what else could it be?