What Causes Missed Periods With Negative Pregnancy Test?

Hi there my name is Lynn Clydesdale I am 22 years old and I have a question. I have been with my boyfriend for 6 months now and I have missed my period twice. I got in October- once at the beginning of the month and again on October 31st but in November I never got it at all. I got my period in December January and February but I never got it this month at all(March). I went to the hospital to take a pregnancy test and it came back negative. I was just wondering if I am pregnant.