I lost my period, but I'm not pregnant. What could it be?

When I first got my period it was the most dependable thing in the world, I could have predicted when it would come months in advance. But all of the sudden this past year sometimes I would go two months in between periods and now its literally been 4 1/2 months since my last period. I haven't had so I know I can’t be pregnant and I am a little concerned. I was told that if I was under weight that I would loose it, or if I was under a lot of stress. I am no more stressed now than I was a year ago, and there’s no way I'm underweight at the 5'9 and 121 pounds I'm at.