My 14 Month Old Has Red, Hard Testicles. He Screams

My 14 month old has red, hard testicles. He screams in pain when i wipe them during a diaper change. It seems to go away for most of the day but then randomly appears at last once a day. This has been going on for 3 days. He had a nasty diaper rash a week ago but has since cleared up. His first 3 diaper changes today were fine but the latest one his testicles were red and hard and painful.