i think the most common reason for not missing periods in your age group is being under weight or exercising too much - may or may not be relevant.
Apparently even being stressed can make them irregular - but good idea to get checked out as it can also be due to things like polycystic ovaries etc.

I don't have many because I have a medical condition where I can't absorb food so I am very underweight - I think it's great!! i don't get cramps, or moody etc