My friend had eczema when she was breast feeding, along with cracked nipples and she said it was the most painful thing she'd ever experienced, including the birth.
I think what made it worse was the rash allowed her to get an infection, so she had to deal with that on top of everything else. She said she honestly thought she was dying, as she developed a fever and everything.
Luckily her mother showed up and got her to the doctor. A few courses of antibiotics and she felt much better, but she was careful not to let the rash get away from her again after that.