I need help with my bra size??:(?

I'm currently 17, and this year my body decided it would be a great time to finally grow! Well, I'm naturally reallyy thin, and up until this year, pretty flat.. anything out of an A cup was pretty daunting! Well, I now have a chest (sadly with some stretch marks, any ideas on getting rid of those would be great to!:D) and it's measurements are: below the breast/band size: approx. 26-27 inches. the fullest part of my chest: 33-33.5 inches and under my arm pits at the top 31 inches. So i used a bra calculator and it gave me a 30C!? Woah woah woah. how could i have bumped up that much so fast?? they don't seem *that* big?? also, i noticed if i change just the band size from 26 in. to 25. it goes back down to 32 A??? so uhm.. what would you say is my size? :( really confused:(