Well, it could or could not be. :>) An X ray is an important part of diagnosing this. You need to see a doctor who will do a physical exam, take a medical history and take x rays. Know that osteoarthritis of the knee does not usually start suddenly but builds gradually over years. You often would have swelling in the area too. Activity makes it flair or sitting for long periods until it is progressed to the point of constant pain. This doesn't exactly sound like you. You are also young for that unless you've had an injury or were an athlete that had a lot of overuse.
Exactly where the pain is around the knee is helpful too. If you are active, an issue with the cartage in your knee could be the culprit.
Again, see your doctor for an X ray. Let us know what happens!