Bra talk!!

So how exactly are bras supposed to fit? I've always worn a 34/36 B and I went to Victoria's Secret yesterday to get fitted and in some bras I am a C but in others I am a B and I just don't quite understand how bras are supposed to fit? My nipples always get smushed by B cups, but C cups seem to be a good fit but then other times it's too big. I have very weird boobs to begin with, I am just very confused! Help!!