Continuing from my question yesterday:

I'm thinking now that it could be yeast.  Although there is no cracking or
flaking, she does say that her nipples seem a little pinker than usual.  I
don't think it's just the fibrocystic condition because she hasn't
experienced breast tenderness before and this is pretty extreme.  I don't
think fibrocystic lumps would cause such sudden and extreme tenderness or
would make her nipples sore.  Neither she nor the child have been on
antibiotics, so I don't know why she would suddenly get a yeast infection
after all these years of nursing two kids, but I can't think what else it
could be.

-Lucy