What Causes Irregular Periods While HPT Is Negative?

I have not had my period for the entire month of january. The last period I had came on December 21 and went of on the 27. Today, February 7, 2015 I am bleeding again. I was reading some articles about how during pregnancy it is normal to bleed after you missed your period. I took a home pregnancy test and it said negative but I am not sure. Can you give me some insight please