Antibiotic and rash?

My 13 month old boy has strep throat, he started an antibiotic on August 12. On August 13, I noticed some small pimples on his upper back. Today (August 14), he has a lot of pimples (all the back to the buttocks, all the belly to the turn of his penis and a little on the face). He had already made buttons with another antibiotic but at the 8th day. Should I consult or should I continue to give him his antibiotic without any problem? Thanks :)