diaper rash (yeast?)

JJ1973

My LO has never had diaper rash (he's 10 months). But today, daycare called and asked if they could put some ointment on his bottom because he had some kind of a rash after two bowel movements. I told them sure and then picked him up and we did our nightly routine. When I changed him prior to bed and before his bath, he was really red. Smooth skin (nothing raised), but really red all over. He's now in bed with lots of butt paste on. 

I'm currently on antibiotics for mastitis and strep throat (yup - got them both this past week). Still breastfeeding. I've been giving LO probiotics in hopes to avoid thrush or yeast issues, but could this be related to my antibiotic use? It is just strange as he's never had diaper rash before.