Late period, not pregnant?

I usually always have my period on day 29 every week like clockwork since I stopped taking the oral contraceptive (it has been 3 months since I stopped taking it). A while after ovulation a had a slight bleed which didn't have the consistency of discharge, and it lasted on and off for two days. Towards the day that my period was due I started to become lethargic, had cramps and sore but still no sign of my period. I took a pregnancy test this morning and it was negative. I am now three days late. I am not under any serious stress and I am not on a diet that has caused me to lose a serious amount of weight. What could be the underlying problem?