Bactrim(sulfamethoxazole/trimethoprim) is "maternal medication usually
compatible with breastfeeding" according to the 1994 American Academy of
Pediatric's committee on drugs,
as reported in the 4th edition of Lawrence's BREASTFEEDING:  A GUIDE FOR
THE MEDICAL PROFESSION, page 707.  comments on sulfamethoxazole are "In
milk:  about 1.2% of newborn dose.  In first month, displaces bilirubin.
Contraindicated in G6PD-deficient infants.  Cow's milk: 50ppb. "

Might want to inform mother and the physician of this!

Natalie Shenk BS IBCLC
Ohio USA