This sometimes happens to me!
Do you clean baby’s mouth? Could be thrush
Mine got like that too! Never understood why.. I never got pain or had any problems expressing milk (I would express/pump then bottle feed) I just took it as my ducts were open and my nipples we’re getting over stimulated in a way? It does go away though, well for me it did.
This just happened to me for the first time two days ago too! I'm still not sure why either.