Antibiotic reaction

My doctor prescribed me Keflex for an infection in my csection incision last week. "Safe for breastfeeding". What she didn't tell me was it might cause gas and diarrhea in my newborn. He had a diarrhea diaper yesterday and one this am. No other signs of him being sick - plenty of pee diapers and eating like a piggy. My Facebook mama group said many of them had this with their babies when they were on antibiotics. Why does no one tell you these things?!?  Also explains why he's been spitting up more. Thank God I take my last pill this am.