My 10 month old injures himself...?

Everytime I change his diaper or give him a bath, he squeezes and twists and scratches his testicals so hard that he gets them all red and I'm afraid he will injure them badly. I've tried to distract him with toys and books but nothing deters him. He screams when I put his diaper back on. He is always dry and clean. No rashes. Why is he doing this? He is not masturbating, he's violent with them. No funny comments please.