Is it just me or have her boobs been looking pretty lopsided lately? I think I remember seeing another recent photo where I thought the same thing. It's normal to be lopsided and it becomes more prominent when you're breast-feeding....but somehow I doubt she's breastfeeding since she's always out partying and messing around.
A bra would be the obvious answer, but like many obvious things (KFed = loser, bare feet + gas station bathrooms = gross), it seems to have gone totally over her head. Oh, Britney. You are a mess.