I agree with the other posters that this is typically normal. That being said, I would still take her to a pediatrician and have it checked out. A few years ago, I wouldn't have thought anything of it - until my girlfriend's daughter (a little younger than yours) developed a lump on the side of her breast (closer to the armpit). Tragically, it was cancer. A million kids could have that same lump and it would be nothing, and I'd be willing to bet that it is nothing in your daughter's case either. When my girlfriend showed it to me, I blew it off with pre-puberty budding, too. I thought she was over reacting when she took her daughter to the doctor. I certainly don't want to scare you, and I know that I am "over sensitive" to this because of what happened, but it would be worth a trip to the pediatrician just to have a look-see.
God bless you and yours as well.