My knees hurt? What's wrong with them?

I'm only a freshman in high school and at the beginning of the school year way back in about November I noticed that my knees began to seem a bit sore. The soreness went away but it was replaced with a kind of weak feeling. Around December after playing paintball for a couple of hours I noticed my knees hurt whenever I bent them with pressure for example from kneeling to standing up. Well that slowly went away but now it is again replaced with a kind of weakness. Whenever I play paintball my knees hurt when crouching and feel extremely weak afterward to the point where if they were anymore worse they would give out. Nowhere knees get very shaky when I skateboard. Earlier today I jumped off a wall and landed wrong and felt knee pain for about 5 mins. Afterwards my knees felt so tired that I couldn't keep doing what I was doing so i took a break now about 5 hours later my knees feel fine but with a little bit of pressure just below the knee cap. I would go to the doctor but I'm hesitant because I want to join the Marine Corps when I'm 18 and I don't want a knee injury to disqualify me. Lastly I'm sorry if this is written badly as I'm writing this on my iPhone and thanks for answering and trying to help me out here!

Go to the doctor to get your knee check so it could get better before it becomes something serious.