Don't ever hesitate to call a doctor for a really bad sunburn. Sun poisoning is absolutely no fun. I had a friend who was on a mission trip and was on the boat going back to the mainland. Her face and torso were in the shade, but her legs were in the sun. She fell asleep and when the boat docked about five hours later, she was cooked. She had to go to the hospital and when she got back to school had to wear shorts because she couldn't wear jeans. Her legs looked like fried chicken.
I've found that for a less severe case, Aleve or Advil help. These are anti-inflammatory drugs and they really do give some relief. Put the aloe vera gel in the fridge for an hour or so before putting it on the sunburn. It's the best treatment, and when it goes on cold, it's almost painful because it feels so good.
Wear sunscreen. The best way to treat a severe sunburn is to prevent one.