What Causes Delay In Menstruation Other Than Pregnancy?

i havent got my period in 3 months. On my 1st month I had a spotting day but that was the last i saw of my period. I took 2 pregnancy test both came out negative. I have been sleepier than normal but thats it. Since I started my period when I was 13 I had never missed my period, sometimes a day early or a day later, but never more and im 20 years old.