What Causes Delay In Periods Inspite Of Negative Result For Pregnancy?

I am not on birth control but my period has been abnormal, not to this extent thought.. My last period was 56 days ago. It only lasted 3 days. I have had intercourse on firtile days. I don t have any visible simptoms of being pregnant. I have taken 2 tests last week and both were negative. In November, my period was about a week late. That s the most its ever been late. I don t want to get my hopes up. What could be happening?