I had 6 naked girls ranging from 3 to 6 running around in the backyard on Friday. Some had the little fatty boobies you describe, some didn't. The ones with a belly seemed to have them more than the really skinny ones. Girls tend to get a belly before a growth spurt ie they seem to lay down fat then shoot up. She may be getting ready to do that.
I agree with Nai that maybe she's just about to shoot up. Sometimes I look at DD adn think she looks a bit chunky (horrid mother that I am!!) and then the next week I look at her and she looks heaps taller!