Sounds like a breast bud. If you're concerned, have her see the pediatrician, but I'd bet it turns out to be a breast bud, which, at this age, is completely normal.
One of my daughters developed a breast bud at 7, so we did some testing to check for early puberty. Everything was normal, so the doc wasn't concerned. At 8, her breasts were starting to grow, she was developing hair in her armpits, there was some BO going on, etc. I took her to the ped again because it REALLY was puberty starting. Ped said at 8, it's normal to start these things and not to worry. Daughter got her period at 10 and now, at 12 is a C cup, just like her aunties at the same age, and STILL growing. She DID start early, but not SO early that it raised any red flags.
It's always nice to have peace of mind, so maybe a quick trip to the pediatrician might be in order. At the very least, your doc can have a chat with you and your daughter about the changes her body will go through over the next few years.