15 year old with Irregular Menstrual Cycle- Not ually Active- Had my Period since I was 11, What's Wrong?

I'm 15, but I started my period when I was 11. Within the past few months, my period are have been very irregular. I am NOT ually active (I never have been) and my family does not have any hereditary issues. And I do not have a eating disorder or anything of the sort. Yes, I have been stressed but it shouldn't be affecting my period for four or five months! Starting in November of 2008, I had my period normally (around every 30 days). In December, I missed but had it in early January (45 days). In February, I had it normally- 28 days. I have yet to have it in March and it is currently day 51. One more thing- every year my period misses one time (this happens with my mom too). It usually happens when it changes from summer to fall and so my menstrual cycle changes from about 28 days to 34 days. This is normal in my family, but recently over the past 4-5 months my period has been WAY out of wack.