Have you been to your GP? Periods can be effected by weight gain/loss, stress and diet. I had a similar problem at your age and didn't get it checked until I was early twenties and it turns out I have polycystic ovary syndrome. This can effect your fertility but it depends on degrees of severity. They test for it by doing a staple test on your androgenins (sp but it's early) and if they're high (I think, again, it's early!) then you get sent for an ultrasound or an internal scan (a rather large tampax shaped thing is inserted inside you, it's uncomfortable but not painful) or both. The treatment is often the pill, specifically dianette in my experience. There are other ways of managing it if like me you prefer not to take the pill for an unspecified amount of time.