How to tell you're wearing the wrong bra size, from the New York Post:
--The bra is rubbing (and chafing!) in certain spots. Most people think this happens because your bra is too tight, It's actually because the band is too loose, causing it to move around and irritate the skin.
--The straps are falling off your shoulders. This is also a sign your bra is too big. Think about going down a band size or, in some cases, a cup size.
--You're popping out at the middle. This is a sign your bra is too small --when your breasts do that double bust thing, mashing everything together into a uni-boob. Go up a cup size to correct it.
--The fabric is wrinkling... this is a bigger problem on people who have smaller chests. It's a sign your cup size is too big. Go smaller for a more snug fit.