Hormones, such as those associated with puberty, pregnacy, etc., can stimulate the grow of benign lumps such as cysts and fibroadenomas. These are not harmful to your baby.
On the other hand, it IS possible to develop breast cancer while pregnant or nursing.
The bottom line is that you should get this lump checked out through a clinical breast exam and additional testing if indicated.
Aside the possibility that this lump could be a cyst or a fibroadenomas, this could also be a blocked milk duct which happens quite frequently when breast-feeding.There are milk ducts that go up near the armpit.
Try nursing first from the breast with the lump, apply warm compresses before nursing and massaging in the direction of your nipple and see if the lump goes away after a few days.If the lump persists,then it's better to have your doctor examine you to also make sure that you don't have an infection.
All the best...