The knee has so many structures involved with it that knee pain can be any number of things or combination of things, not just arthritis. The main thing is to get to your Doc, especially an orthopedist, ASAP and have them check it. It might be something as simple as stretching and strengthening and taking an anti-inflammatory and icing your knee for 20-30 minutes. The problem is, if you don't get it checked and it is just a minor thing, it could turn into something worse. Since you're not a 19 year old athlete that heals quickly, it's even more important. You could have a meniscus problem, patellar tendon problem, the tissue under the knee-cap could be the problem, and the first one is inside the knee capsule, the second one is on top of the knee, and the last one is under the knee-cap but outside the knee capsule, and that's just three of the things it could be. So go get a good knee evaluation by a good orthopedist, find out exactly what the problem is, and get it fixed BEFORE it gets much worse. If it's the worst case scenario and you need a knee replacement, then it's also important to start rehab BEFORE surgery so that you can come back quickly.