Hi Niell,
A couple years ago I had a similar problem with pain in my "vintage" left knee after riding.
I would echo the advice that your first step is to have it looked at by a doctor.
In my case the doctor found no major problems, and the diagnosis was (in lay-speak) my knee-joint cartilege gradually becoming "vintage" too, resulting in bone-on-bone pain in my knee.
Subsequently I put myself on a massive regimen of glucosamine-chondroitin for several months. (A dietary supplement which supposedly helps the body rebuild cartilege, and is available at most health-food stores). The good news is that after several weeks, my knee pain went away, and has not returned since. (Knock wood!)
Good luck -- I hope your knee stops hurting soon!
Steve Goodwin Westford MA
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