Knee Pain in 15 Year Old

Hello, I am a 15 year old boy, about 5 10`. At the start of the year I started going to the gym. I used the treadmill a lot to improve my aerobic endurance. Started at 20 minutes 3 times a week to 45 minutes 4 times a week 6 months later. I also played soccer and cycled, and participate in sports relating to my P.E. course in school. For the past 3 or 4 month I have been experiencing knee pain in my right knee. It started out with sharp pains inside my knee when I was doing anything from walking or climbing stairs to standing up from a chair and aches when I was sitting down for any time longer than 1 minute. I also have a lot of 'cracking' or 'popping' noises when bending my knee. Also when standing up from a squating position there is a grinding or noise like a door creaking, as if the bones are rubbing against each other. I attended a physio and was told that they couldn't find anything wrong with my knee and that it was probably something called Osgood-schlatters disease(sorry if i misspelt) and that I would grow out of it. Since then the pain has decreased considerably I can now walk,cycle and climb stairs without the pain I used to get but I am still experiencing slight pain when playing soccer and other activities. There is still the popping and grinding noises. I am really worried as this has been happening for the past 3 or 4 months and my practical P.E. exams at school have been affected. Could anyone tell me what could be wrong with my knee?