How old are you? If you are still in your teens I wouldn't worry about it too much if you're not sexually active. It can take till your early 20s to become very regular, however, sometimes girls can have problems staying regular in which birth control might be of some help.
What is your activity, diet and stress levels? I went to a 5 week army school and I missed my period while there, missed it the month after i got home and didn't get it until the end of the 3rd month. Even now at 23 my periods get light and short and irregular with the more excersie I get and heavier when I'm sedentary (injuries).
If you are really worried about being pregnant, just get a pregnancy test at the drug store if it'll help your peace of mind.