ok first of all: stop yelling. turn off the caps lock key.
second of all: why the heck would you have sex at age 13??? you're my age, for lord's sake, and i've never even kissed a guy.
third of all: you're young. you've probably had your period for what, a year? it can sometimes take up to 2 & 1/2 years to become regular. you may just be having an irregular period.
fourth of all: if you DO happen to be pregnant, don't get an abortion!!!!!!