My mom and my period? Why does she do this?

Okay so I have a small basketball practice for a 3 on 3 tourney we are having soon, it's from 1:30 to 2 and we are just going to shoot around. Then we are going to my friends to make our shirts, which will take a couple hours. I started getting period cramps about 45 minutes ago. I just went and told my mom and she flipped on me. She started yelling saying I'm just nervous and all of this crap, when I'm not nervous cause they are 3 of my really good friends so there isn't anything to be nervous about. I told her I usually get cramps on the second to last day and she yelled AGAIN saying that's not normal and that people usually get them at the beginning of their period. Is this true? Cause I've always thought every girl was different? But it's wrong for my mom to yell at me about this, right? I'm only 13...and everytime i tell her I have cramps she yells at me...and it makes me cry, like I am right now. She's usually so supportive, why isn't he when it comes to this?