What Causes Delayed Periods, Hard Stomach And Mild Cramps?

My partner and i had sex on the 26th of December and the condom split i look morning after pill and had jan and feb periods as normal was due one at beginning of march which was 1 day late and heaver than usual and i was due another period 4 days ago, but there is nothing, I have had mild cramps and my stomache is hard but not my usual signs of due period. Ive taken two home pregnancy test (first thing in morning) and they are both negetive. What could be wrong?