For awhile I have a white lump in the tip of my nipple, under the surface. Not growing, not going away. Help

This is not like a pimple, it is hard and becomes whiter and more visible when pressed on. The nipple isn't deformed by the lump. I had a reduction three years ago, and was feeding up until 9 months ago, but had no milk production from this and consequently only fed off the other one. Any clues as to what this could be?