Hi there. I was 12 when I started getting my period and it definately wasn't regular...sometimes I'd go for three months without a period (wasn't sexually active) and sometimes I'd get it three months in a row, for a while I regularly got it only every other month. I might be your body adjusting to a new experience. I think it's pretty common for your body to be pretty irregular for at least the first year.
I'm 24 now, but a few years ago when I was about 21, I had a really stressful summer and I rented a room in a house that had about 3 different women in and out throughout the summer...I actually skipped my period for about 2 months! I think this was due to stress and being around other women. One thing you'll find is that some women's body's tend to "lead" other women's. So, for example, a woman who has a really strong "lead" (I'm not even sure what to call this) will often disrupt the flow of others. For example, I moved into my current house 6 months ago and I noticed my periods were regularly every first Saturday until one of my roommates moved out, at which time, it became sporatic again. So, you're body might just be a bit confused right now...but if you are still worried and concered, I'd suggest to see a doctor, at least for your own peace of mind.
I hope this helps