It could be a clogged milk duct. Try putting a warm compress on it or take a hot shower and then after a few minutes massage the lump. While you are massaging move your fingers from the lump down towards your nipple.
Watch for any flu like symptoms and a fever as if it is a clogged milk duct it can become infected.
If the lump doesn't disappear I would get checked out by your doctor
I am not breastfeeding so I dont think it would be...??
It maybe some type of cyst I would go and have it checked out. They may put you on antibiotics or have it drained. It could also be an infection I guess that is not uncommon in pg women. I would call your dr definately. Best wishes to you and good luck!