On page 429 of the ever-useful Breastfeeding Answer Book, under the
heading "Deep Breast Pain in the First Six Weeks," it mentions muscle
strains as a possibility.

A more likely explanation -- "Pain from a forceful let-down may be due
to the stretching of the ducts as the milk passes through them."   This
may be worse in an oversupply situation.  The book goes on to say that
"This type of pain usually clears up spontaneously within the first
months of breastfeeding," probably just because the ducts become
accustomed to the filling and emptying, and the supply gets into balance
with the baby's needs.

Of course if the pain persists and gets worse/more consistent, you'd
want to look at yeast and other possibilities.  But perhaps the pain
will seem less troublesome during this adjustment time if she has some
idea of what's causing it, that it's within the realm of normal, and
that it will resolve.  Unexplained pain is more painful, to her and her
concerned husband.

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