K.S.
First off listen to your OB. A lot of non OB doctors prescribed medicine they think is safe when it is clearly not. Also I was prescribed Bactrim for a killer UTI when i was pregnant with my youngest. Before I ever even took it I called my OB who was in shock that a doctor prescribed this to be in my last trimester. After reading up on it I found that it is dangerous to take while in your last trimester due to brain developement in your baby. So i am almost postive its not safe while breastfeeding. Also if your doctor thinks you have MRSA has he/she sent you for any tests. Thats pretty dangerous stuff and you have not been tested i would do so right away.