Sounds like it may be a galactocele, which is a milk-filled cyst. They usually occur when the milk duct gets clogged with protein debris. It's probably a good idea to get an ultrasound to check and, if that is inconclusive, a fine needle biopsy might be recommended. If it is a galactocele you can either do nothing as it won't do any harm, or have it aspirated or excised. The main reason for having active treatment is if they are interfering with the baby's latch.
I have one in about the 4-5o'clock position under my areola, if it had grown bigger it may have made latching a problem but, so far, it's been fine. If you have another pregnancy and breastfeeding again you may notice a change in size.
Mother of two spectacular girls, born mid-2010 and late 2012