My 5 Year Old Was Prescribed Biaxin 6 Days Ago

My 5 year old was prescribed biaxin 6 days ago to treat pneumonia that did not get better with a 10 day course of augmentin. Tonight as I was getting him ready for bed I noticed a rash in both armpits which he complained was itchy. I gave him the age appropriate dose of benadryl to help, wondering if this is a reaction to biaxin?? Do I need to worry about possible interactions or side effects?