So, we need more information. I guess my thinking was this was someone presenting to the ER, or family doctor, a mom of an older baby or even toddler--not a mother who had just given birth. The OP was a bit ambiguous, but if the patient had just given birth, that is a rather important piece to leave out, since then there they'd be the possibility of it being a hospital acquired infection. But for a non-neonate breastfeeding mother presenting from the general community to the ER (or primary physician), I still vote for Bactrim as being the drug of choice.