I'm a very athletic person. I injured my right knee a while back (5 years) doing gymnastics. The doctor and x-rays showed nothing. But it still acts up once in a while. This time it is extremely painful. Its as though it is coming from the lower front part of the kneecap all the way in to the centre of that area.
I injured it while bouncing on a trampoline this time, the original injury is from bouncing on an incorrectly set (for my weight) beat-board. Going to doctor's tonight, but if nothing comes up, this is my last hope. They say its growing pains. (I'm a teenager, female) I do not think so, I'm pretty much at my estimated height, if not taller, and growing is pretty much accounted for. ANY idea as to what it is?
Thank you!

Just a guess from the location you say would be that the synovial fluid has been leaking out of the sac. This usually can be heard, however. Creaking and popping and grinding noises. But sometimes it is 'just a nerve' that is pinched or irritated or being pressed on by a swelling or by scar tissue. And it's an enormous about of work to identify which one.
Another condition is called chondromalacia, which you can look up. I would think that you might be too young for that, but maybe not.

You need a good sports clinic and you need more than an x ray. MRI or PET. Your doctor should not be so inattentive to real pain, just because nothing shows on the x ray. X rays don't show a lot of what goes on in a knee.