I'v had three knee surgeries AND stayed at a Holiday Inn Express last night so......Did anything hit you that would cause a contusion (bruise)? Is this located anywhere near the knee? If you did not hit/be hit by anything, and it is not in the vicinity of the knee, it will probably be a soft tissue/muscle injury of some type. Either a pulled muscle, and a severe one with bruising, or a torn muscle probably in your quads if in the front of the leg. Any "pulled" muscle is really torn-it's just a matter of the degree of the injury. Grade one is a pulled muscle, grade two is a partial tear of the muscle, and grade three is a complete tear that could require surgery. Was there a specific moment that you remember causing the injury? The doctor will press you on this if you have to go. Take anti inflammatory meds like Aleve or Motrin, ice (20 minutes ever hour if severe), and elevate if swollen. Sorry so much info, just had my more than fair share of sports injuries. Absolutely stay hydrated as Skinsmarydu said. You need to get off the leg and if any other new symptoms appear go get it checked.