This is along the lines of mastitis cause by a blow.

Use of a sling caused recurrent plugged ducts and mastitis for me.  This
occurred when I carried my baby for hours.  I preferred carrying the baby
on one side and so usually kept the sling on the same shoulder.  If I
didn't watch the sling position carefully it would compress the breast on
the side where it came over my shoulder, resulting in plugged ducts and
mastitis.  It took me some time to realize just why I kept having problems
with that breast!  Once I did, I was able to prevent recurrences by
insuring that the sling did not compress my breast, and use of breast
massage anytime I carried my baby for a large amount of time.

Debbie Owens LLLL