What Causes Missing Periods, Abdominal Cramps And Negative Pregnancy Tests?

hi doc,i had my period on the 25th of October 2011,i was expecting it on the 22nd od November 2011 but its over a dat now and i have not seen it.i did a pregnancy test on the 18th and 19th of november 2011 but both results were negative.now i have abdominal cramps.have a feeling i was to menstarte but nothing is in yet.what is happening to me.i also feel movement in my navel.