Ummm how do i tell my mom to get me a bigger bra?

wellllllll, my birthday was yesterday but i slept through it (hahaha) because of testing that day. so i only got my laptop and a mouse for it. i kindaaaa snuck a peak at my step moms present and she got me a training bra and a padded bra. i kinda been weearing a padded bra thats a 34 a but havent told them. i wear a hoodie over my shirt then take it off at school. hahahaha. but i just tried on the bras she got me because my parents are at dinner but theyre a little small. theyre 30 a. she said im going to get my present at my party at boomers (south florida and CA thing) but i dont want to be like "oh, ummm, laurie? i need a bigger bra" also i do my own laundry so she doesnt knowww. ummm what can i tell her????? thanks!