Sounds like a milk blister, aka bleb. It's basically a clogged duct but the duct is right at the surface of the nipple. Does it hurt? Warm compress and nursing is the best cure but since you've already weaned I'm not sure what would be best and whether you even need to worry about it. I would imagine that if it doesn't bother you, it probably isn't a big issue.
Because you're no longer nursing, the standard things wouldn't make much sense (like nursing often, pumping or warm compresses). I imagine your goal is to let the milk dry up completely. You could take lechitin supplements but again I don't know if that would help since you're not removing milk. I would simply keep a close eye out for clogs/blebs and deal with them if they come up.