My Son, Age 10, Has A Rash (raised Red Large

My son, age 10, has a rash (raised red large bumps that itch) that have come and gone for over a week. They appear on his legs and stomach. Benedryl helps relieve the itching but we aren't sure what it is. No fever or other symptoms. Thoughts? I was thinking something possibly related to the recent warm weather or excessive pollen?