Nutrition might alsoo be the issue. Last summer I was really stressed and not eating a whole lot and working my butt off. I worked out about 3 hours a day and lived on nutrition shakes. I didn't have my period for 4 months!
I talked with my Doctor about it and he said I didn't have enough body fat and that was why I wasn't having my period. I started eating more healthy fats like nuts and sure enough my period started back up. About 6 months ago I got on birth control for my skin problems and it has done wonders for me! No more break outs, no more mood swings and reliable periods!
If you do decide to get on birth control pills make sure you really research them. The kind I used was actually invented to help skin problems and being a contraceptive was actually just a side effect of it. Others can be dangerous though and cause cancer so be careful!