I wouldn't be too concerned just yet. It is common for periods to become irregular if one loses weight and ups their activity or exercise. As an example, some female athletes will stop their periods altogether when going through intense training. And, well, my own niece didn't start her period for the longest time! But, she was a slip of a girl; probably only weighed 90 lbs. Once she started to put on more weight, THEN she started.

As for myself, I didn't have a period for one full year. There was a reason for that and basically due to severe stress. I was around age 23 when that happened. So, I went to a Reproductive Endocrinologist; actually an Infertility Specialist who ran all kinds of tests and nothing was wrong. So, and I forget what the shot was, but I believe it was Progesterone and that triggered it. It was just way too dangerous to go that long without a period as the endometrial lining just thickens. Anyway, that was an extreme situation. But it was explained to me it had to do with the pituitary gland and a long story.

I would say if you don't start soon, get with an ob/gyn or RE who can help getting you straightened out IF there is a problem.