Jo Ann Gerling writes:
<<once thrush is ruled out, I can only conclude that the pain may be caused by
the flow of milk dilating the ducts.>>
The reference might be considered old by some standards but Jan Riordan talks
about a cramping sensation in the breast following let-down.  She described it
as “refill pain.”   It is thought to be caused by the rapid refilling of the
alveoli following let-down (Riordan, Jan .  "A Practical Guide to
Breastfeeding."  New York; C.V. Mosby, 1983, pg 50).
While I agree that << heat will help to dilate the ducts and will be soothing
to the mom.>> I think it is important to remember that moist heat will dilate
while dry heat will contract.
Marie Davis