Knee pain is a common complaint. Especially with young healthy and active people, this can be troublesome. This pain can result in significant limitation of mobility and therefore warrants evaluation. I would recommend that you go to your primary care doctor
in order to have this evaluated.
The knee is a complex joint. It requires significant mobility, but also requires a lot of structural support for the entire weight of the body. As such, there are many delicate and intricate parts that can be injured and cause pain. The most common type of pain in a person your age is inflammation. Inflammation can occur with repeated use, especially if the alignment of the important structures is not 100%. The components of the knee that are commonly inflamed include the tendons, ligaments and muscles.
Patellofemoral syndrome is the most common type of pain that you describe. If the knee cap, is a little of balance or not held in the correct spot, then it can rub against the knee and cause inflammation. This occurs after activity as you describe.
Another cause of inflammation in the knee is referred malalignment from the foot. Often inappropriate shoes can cause this.
Talk to your doctor. This requires evaluation. Good luck!