My DD has had left knee pain for about 2 years. I've always given her the ol' "suck it up kid, you'll be fine". Ice and ibuprofen and we kept going forward. This winter she again began with the knee pain line. Okay, says I, and took her to a sports ortho the other day. Doc says that he is seeing the same thing with increasing frequency in female athletes, catchers in particular.
Here's his explanation. As they grow their hips widen. The knee cap wants to move outside in order to stay in line with a line drawn from the top of their hip bone to the lower part of the knee. As they flex and extend the leg the knee cap moves left and right, under stress, causing the pain under the knee cap.
His solution is a certain brace which holds the knee cap in place when she squats and stands. She also needs to strengthen the inside quad to help pull the knee cap over.
This is the reader's digest version but I wanted to share our experience in case any of your DD's are having a similar issue.