I haven't had my period in months, and my breasts haven't really grown much. Any help?

Okay, so I know I'm not pregnant, because I'm only 13. I've had my period for about a year, but 4 months ago, it just stopped. I haven't had one since. Also, my breasts have stayed the same size as they were a year and a half ago. What's wrong? I haven't told my mother about this yet, and it's worrying me. Please help.