It shouldn't be a concern. Cephalexin (keflex) is only minimally secreted into milk, which is why it's considered a good choice for breastfeeding mothers. As far as bacteria, mastitis isn't always caused by a bacterial infection (a blocked duct can cause the same symptoms whether or not bacteria are present); it's usually "standard of care" to prescribe antibiotics whenever a breastfeeding mom presents w/ symptoms of mastitis because there's no good, reliable way to test if her symptoms are caused by bacteria or not. And if bacteria are present, most likely the bactera were introduced from baby's mouth or mom's skin, which means that both you and baby normally have them present and are already exposed to them, so feeding milk pumped during a breast infection isn't a concern. Your body, your baby's body, your milk are not sterile and shouldn't be. Bacterial levels in human milk actually decrease several hours after pumping (those bacteria aren't actually "in" your milk but are on the pump equipment or your body and get into the milk as you pump) since your milk contains active antibodies.Â