Diamond,
13 is very young to be concerned about being pregnant. If you feel you may be, pick up a test from your nearest drugstore. At 13 your body will still be getting used to the hormones and irregular periods are VERY common. If you are worried about being pregnant, I would really suggest using effective methods of contraception. Condoms are really easy to get and fairly inexpensive. Just please use caution, I hated hearing this too, but you are still so young and sex is a huge (and I mean HUGE) step into adulthood. Enjoy your youth as long as you can, you really only get it once.
If you have any more questions or concerns let me know. Good Luck!