Welcome to Just Answer and thanks for your question. This is probably an overuse injury. These are generally treated with ice, rest and anti-inflammatory medication like Advil, Motrin, or Aleve. When applying ice, wrap it in a washcloth or thin towel and apply for just 5-10 minutes at a time several times a day for 2-3 days. Then stop. The idea is for it to be cool, not frozen as this could damage it worse. I would suggest no Guitar Hero for a week or two. If it isn't better in a week, you should see your doctor.