How should I fix my knee pain?

I'm a 16 year old male and I've had this pain in the back of my right knee for a 3 months now.The pain started off as a slight ache and did not occur after any major accidents. The pain is almost instantaneous and usually occurs when I put some sort of stretch on my knee. Mainly, when I lye down to sleep or when I do stretches for pre practice. I've told multiple people about this and they have told me to stretch it out and it should loosen up. However, It only gets tighter and more painful as time goes on. Now I'm no stranger to injuries, I've been playing contact sports like football and wrestling for 3 years now. However, this occurred when I started playing basketball, a relatively non contact sport. There is no pain while walking or doing physical activity only when i put strain on my knee.I have also previously injured this knee 3 times before with an injury that I don't feel is related. However, if it helps, the previous injuries started with a pop in the side of my knee followed by about a 3-4 week recovery from tendon damage. I would like to get this out of the way as quick as possible. Thanks in advance for your help!