What Causes Abdominal Cramps And Late Period?

OK, I according to my calendar was supposed to start my period on Sunday, it didn't come but I was experiencing cramps like I normally would pre period. I was not experiencing breast tenderness like normally though. Today is Friday and I still have no period. I took a pregnancy test yesterday which came back negative. My breasts started hurting this morning also. I also have low back pain. The cramps stopped Wednesday. I don't understand what's going on...can you suggest anything?