Why My Period Is Delayed With Sore Breast And Back Pain?

My last menstrual period was on 12/3/2010. on the 14 I had sexual intercourse with my bf and the condom broke. the very next day I took the morning after pill. My breast swelled and were sore on the 26 and continue to be sore but not as swelled. I expected my period on the 31 but it has yet to come. I have light cramping and some back pains. I took three pregnancy test 7 days after my missed period and they all were negative. Could I still be pregnant?