What cause a lump on an infant breast?its only on the left side.

Usually hormones: Infants of either sex may have transient nipple/breast enlargement on one or both sides. It reflects maternal hormones that came in and were removed through the placenta until the cord was cut. Remaining hormones may produce this and or baby acne. Rarely if someone manipulates the breast the ducts will open allowing germs in. If it is warm/red/swollen it could be an abscess that needs treatment.