Lump under DD2's nipple

[LimeSlice]

I discovered while feeding her, that DD2 (11months), has a small hard lump directly under her left nipple.

It moves under the skin, is about the size of a Smartie and it is hard.

Is this something to be concerned about? It isn't red, inflamed or giving her grief that I can tell.

ETA: Just felt it again, it is probably fatter, rounder than a smartie...hmm