Prevention of Severe Knee injury

I'm 23 and have been worried about my knee condition. I've always been involved with athletic events. I can remember have minor pains in both of my knees in my late teens and got them checked out. The Doctor seemed undetermined. He just told me to stretch my knees/legs before events. For the last 2 years I've been actively involved in running, and my knees seemed to give me minor pain when I would begin running, but sometimes would subside after 20 minutes or so. Recently however, one of my knees gave me severe pain when I was running. It felt as if it needed to be stretched (loosened up) very badly. I had to walk back to my destination. I rested for about 3 weeks, but when I tried to run again, this severe pain in one knee came back. I'm worried about this as I dont' want to ignore this and have it degenerate areas of my knee or develop into artheritis. I'm not sure if it could be a meniscus tear or not. I'm just writing for suggestions and because this has me very upset and worried that it could develop into something more serious. -Thanks so much! Mark Allen