Why might a fairly active 21-year-old female sometimes get knee pain?

Anterior knee pain or patellofemoral pain is the most common cause of knee pain in fairly active 21-year-old females. This can be caused by tight thigh and hip muscles, so it is important to do a good knee and hip stretch before participating in any activities to ensure that the kneecap is tracking well and prevent occasional knee pain.
 
Transcript: "Why might a fairly active 21-year-old female sometimes get knee pain? Well, I like the term sometimes, because that means that it's not happening with relative frequency. Because if it was when you're active or playing a sport that I'd be more concerned about something going on inside of your knee like a cartilage injury or meniscal tear. But if you're sometimes getting knee pain and it's non traumatic, then the most common cause in females event is anterior knee pain or patellofemoral pain. When we say patella, we mean kneecap. And if you can see here, I've pulled up my tights, but if you can see there's my kneecap right through here, and this is my patellar tendon. And if I straighten my knee and bend my knee, you can see that kneecap tracking. I'll give you a lateral view too. You can see that kneecap is moving back and forth. So if the muscles in the anterior aspect of your leg, the thigh muscles and your hip muscles are tight. For example, you spent a long time studying for a midterm or final, or working on a project at work, those muscles up here in the quadriceps and the hip flexors can get tighter, and that can cause an alteration in how your kneecap tracks when you're playing activities. So make sure that before you participate in your sport activity, you're doing a really good knee, quadricep stretch, and hip flexor stretch to be able to make sure that your kneecap is tracking well, and you can avoid that occasional knee pain."