Can somebody tell me?

Okay, so I'm posting this anonymously. 
I just bought my first VS bra yesterday, and I was fitted. The girl told me I was a 34D. I got home from shopping and to only know/findout that the bra I got was a 32D. It did not fit. 
I took it back to the store today and got an exchange, now however this is a sports bra, how the hell is this supposed to fit? I'm not use to wearing sports bras so I'm not familiar with it. 
Any advice will help.(: