I'm a teen w/ pain under knee cap and inside my knee, what could this be?

Patellofemoral Pain: Your pain can be coming from a variety of causes, but without a distinct recognizable injury that you can recall, the most common cause is a constellation of disorders relating to the cartilage of the undersurface of the kneecap or "patella." often the patella doesn't track evenly in the knee causing inflammation, and softening of the cartilage. This can be diagnosed by an orthopedic specialist.

Chondromalacia?: Probably chondromalacia of the patella, a "softening" of the cartilage on the back of the kneecap. Usually caused by abnormality of the alignment of the kneecap. You can use a strap, such as a chopat and do exercises to strengthen one part of your quad muscles but you should first get a diagnosis and then get specific instructions for exercises so that you do not make it worse.