Diaper Rash Remedy

I have mastitis and am on an antibiotic. My Dr said it could give my baby diarrhea since I'm breastfeeding. Well it did give him diarrhea and we noticed he was more fussy when he pooped since I've been on it. Today I noticed his butt was pretty red and looks super irritated around his butthole (sorry tmi). What should I use for it? I have 3 more days of the antibiotic and I feel so bad.