Hi Rachel,

 Have food sensitivities been ruled out? 
 Nancy Mohrbacher suggests less common causes of a nursing strike include 
sensitivity to a food or drug the mother or baby has ingested (including 
vitamin or mineral supplements, fluoride drops, dairy products, or caffeine)
http://www.lalecheleague.org/NB/NBNovDec92p173.html

I would also consider chiropractic care. The Children's Chiropractic Research 
Foundation site mentions the following  case study:

Infants who have trouble suckling may benefit from chiropractic care. 
           

 6-month-old boy  had experienced an aversion to suckling for 4.5 months. 
Physical examination revealed cranial and upper cervical vertebral 
subluxations. The patients’ aversion to suckling immediately resolved after 
receiving his first chiropractic adjustments (to the occiput and atlas), and 
did not reappear. 
http://www.icpa4kids.org/research_children_chiropractic_breast_feeding_difficu

lties.htm

Kathleen Fallon Pasakarnis, M.Ed. IBCLC
Nurturing Family Lactation and Parenting Services