Possible knee injury

Hi!
I just had to do the wise thing and cancel a ride today due to unexpected knee pain. I became aware of a burning sensation in my left knee while pedaling uphill. It seemed to get worse as I rode on and seems to be based around the inner knee area, radiating down the calf. It's not a sharp pain but more of a burning sensation.
I'm already aware what to do in these situations so I'm going to rest my knee from cycling for at least a couple of days and see how it goes. I could also avoid hills for a time if I'm able to resume riding again.
However, I confess I'm a little worried. Is it common to have such a knee complaint? Could this be an actual injury or is it just a warning sign that's telling me to ease up?
I'm just hoping that a day or two of rest will be sufficient for the pain to disappear but, if it doesn't, I'll have to see a physio.
Anyone have any idea what my problem might be or anyone suffer this kind of burning pain that runs through the inner knee down the whole leg?