Is she teething? I went through awful pain when my oldest was getting a couple of teeth. I spoke to someone on the ABA helpline and it sounded just like thrush so I tried treating that then I realised after about a week or so that it was his new tooth! It was rubbing my nipple and made it SO sore. Just thought I'd throw the idea at you as I was surprised and with your redness I wonder if it's the same thing.

Oh, and it didn't happen with the next baby. Guess it's because they all have different shaped mouths and attachment so no reason why it can't happen now with your 4th.