It takes your body a few years to get used to having a period. It may take a while for it to become normal. The key is to listen to your body. When you start cramping and having other signs, it's time to put in a pad.

Because you're only thirteen, and probably haven't had your period very long (?) it's just going to be irregular. I got mine two full years ago, and it still hasn't gotten itself on a schedule yet. Don't worry about it, it'll come. (As long as you haven't had sex, that is... :P) Anyway, mine sometimes take as long as three months to come back, so yeah. You're alright.