I'm 13 where it seems as though I'll have my period in the next month. but, my mom didn't have hers until she was 15! so what's going on?

Actually, how would you know when you will have your first period if you never had it?

That's okay I started mine at 12 and my mum did at 17 so don't worry I don't know why but nowadays all girls have periods earlier than their mums!
Don't worry it's jus normal! Good luck! You got annoying cramps and smelly sanitary items coming your way lol!