Breasts do shift depending upon what posture you're holding at the time. If you are in the United States and live near a major shopping mall, go and pay Nordstrom's a visit. A former girlfriend of mine used to work there and they have the best bra fittings around. Those ladies know their stuff! Many women often purchase bras that are the incorrect size or shape for their bust.
As a younger girl, get the advice of those ladies who have been at it longer than you have. You can't go wrong with Nordstrom, I've sent several female friends who have complained there too and they've always been happy with the results. If you have no Nordstrom nearby, The Gap does them as well, but generally speaking their staff isn't quite as knowledgeable.
I'm currently wearing my favorite bra which I had fitted in a store. I am laying on my bed and I have the same problem. Like @TheDigitalSaint said your boobs shift depending on your posture. Maybe get a better quality bra or try a different size. Maybe your not fitted correctly. Since your a smaller cup size, maybe try a double padded bra or a pushup bra. I am assuming this is one of your first bra, so I'm gonna tell you now, the business of having boobs is a tricky one, very few women ever find the perfect bra. If you think if fits fine, and it feels comfortable, then don't worry about it.