I have seen tears without specific trauma in athletes your age. A strain of either could cause pain. There is a muscle that runs across the back of the knee, called the popliteus. Runner's knee often causes pain in the back of the knee, even though the problem is the kneecap. Most of these problems should improve with appropriate physical therapy and rest.
Persistent symptoms raise concern about a meniscus tear.
You should follow up with your physician for further evaluation.