1 year old sunburn, rash, allergen?

My baby is 11 months old and like me he has super fair skin and yesterday we were outside for probably an hour but he was wearing a long sleeve shirt (it was sunny). Today I noticed his arms were very red and warm to the touch but no bumps. He’s not acting any different and he didn’t try any foods that he’s never had so not sure if it’s a sunburn or what? Maybe heat rash?

His doctors office only has an emergency nurse line and I don’t think this is an emergency. He doesn’t have a fever either, acting totally normal.

Just curious if anyone had thoughts on what it could be