Haven't gotten my period in a while?

I'm turning 17 in a month or so. My period keeps going on and off. Sometimes I only get it for 1-2 days and then it disappears for like 2 months and more. However I do get stomach-aches/cramps, but no period. It's been like this since the first time I got it. Also I've never had so I can't be pregnant. Should I be worried?