Carolyn, I do agree in part that there can be an emotional basis for unexplained nipple and breast pain. However, when we hear hoofbeats, we need to think of horses, not zebras.  Most early nipple pain is trauma from a poor position and/or poor suck, and that affects milk transfer and therefore the baby's food supply. The biggest clue to trauma is the nipple distortion immediately visible post-feed. Most late-onset breast or nipple pain is infection from bacteria or fungus. The big clue for infection is "sudden-onset." 

Once trauma and infection have been ruled out and/or dealt with, what's left can have many causes including emotional trauma. Emotional causes for nipple or breast pain in the absence of trauma and infection are the "zebra" causes. In my experience, many women who were traumatized do not even START breastfeeding. (That's a big deal by itself!) While emotional causes of pain are not the first thing to look for, they certainly should be on the list of possibilities.  The bottom line is that ALL causes of nipple or breast pain during breastfeeding should be carefully and thoroughly investigated.

Linda Smith, BSE, FACCE, IBCLC
Bright Future Lactation Resource Centre, Dayton OH
http://www.bflrc.com/bflrc.htm