Had my period last month, but a week after my period i was spotting, had sexual intercourse. now my period is late. what is happening?

Check pregnancy test: Because you're late, check HPT. Irregular menses are very common in young women. Check out http://en.Wikipedia.Org/wiki/anovulatory_cycle for info re anovulatory cycle as cause of irregular menses. Hormonal birth control pills can be used to regulate menses but don't prevent sexually transmitted diseases. You'll need to make your partners wear condoms each & every time to lower infection risk