I have a 15yr old 6'2" daughter that is complaining about pain behind her left knee that goes up her leg and feels like a cramp. i pressed lightly on the back of her knee and there is a weird sensation like little bumps under the skin and there is a sound

Doctor can evaluate: A teen with some pain behind the knee, and a weird-feeling "something" there, may have inflammation there (called a baker cyst). Other problems can occur, so a primary care doctor, a sports medicine doctor, or an orthopedic doctor can evaluate. Often with active teens, knee problems show up when sports training increases, when a new sport is started, or new shoes are used, etc...