I think mostly everyone would answer your question with "Depends." It could be a severe form of arthritis that is just manifesting in your knee first, or it could be nothing a lil cortisone shot can't cure. Hmmm that's a double negative and makes little sense even to me who hopefully understands what he's trying to say before he says it.
Not sure about the disability thing. I know in canada, if I tried to file for disability with the only symptom being knee pain, I would probably be laughed into outer space. Actually I might be able to get away with it if I listed my current occupation as "kneeing people in the face, groin etc." Here it's all about how the disability affects your ability to do your job. Might be different where you are.
-SLE, RA & PA....it's all about the acronyms!
-Current meds: Plaquenil, Sulfasalazine, and a little NSAID du jour for flavouring. Mmmmm NSAIDs.