This sounds like a breast bud. Breast buds are actually the beginning stages of breast development. They are usually round, and located precisely behind the nipple (if it is off center, then other causes of lumps need to be considered). The actual term for early breast bud development is premature thelarche. This can be on one or both sides and can happen to girls anytime between birth and six years of age. While not considered normal, we also don’t consider this a disease. Most cases of early breast development are harmless, and do NOT progress to actual full size breast development. Your doctor should check for other signs of puberty: rapid growth, acne, pubic hair, and menstruation. If these signs are present, then a full hormonal workup should be done.
Why does this happen?
Breast development is stimulated by estrogen, but studies of young girls with simple premature thelarche show normal estrogen levels. Most physicians believe that some girls are just temporarily more sensitive to their NORMAL levels of estrogen.
Any treatment needed?
Usually no treatment is necessary. Since there is a very, very small chance that this is the actual beginning of puberty, it is important for parents and physician to monitor the breast buds for significant progression.