Stopped bf last week and have hard bit under one nipple.

[quazi]

Does anyone know what this is - one nipple has really hard bit, feels almost like scar tissue - and it is tender and sort of stings to touch. Looks normal on outside. Thanks!

It could be a blocked duct.
I managed to clear mine up by massaging them until the lumps dispersed.

Yes, I'd say it sounds like a blocked duct - try using the shower head to gently massage the area with water, and hand-expressing a little to unblock.