My Sons Sack Is Red And Hurts Him. When He

My sons sack is red and hurts him. When he was in the bath just now he was crying and wouldn't sit down in the water. When I went to go wash him he started screaming and shaking. His stomach was tight. I could tel he was in a lot of pain. It doesn't look like a diaper rash. It looks like there is a bite on one of the balls. Why could it be? Should I call his pediatrician?