Been having some difficulty with this for a couple of months, after no trouble whatever for about 30 years of cycling.
Odd buisness, mostly the complex of tendons at the outside of the right knee. Does not seem to be bothered by low-effort spinning, but climbing will irritate, as does walking up stairs. Feels like the whole area is tying up, with pain radiating both up and down.
I suspect some sort of tendinitis, but I can't imagine what would be irritating the outside of the joint like that.
Go to a Sport Massage Therapist and tell them to do muscle release session(s) for ITB Syndrome (Illio-Tibillial Band). A common (Hip/leg) not actually a knee problem, but will 99% of time feel like your knee is the culprit. You would swear you had a knee problem, but it's the ITB Band tightening up and pulling at the ligaments, etc... on the outer part of the knee joint... right at /in the joint area. Specilized pre and post ride stretching, sports massage, Ibuprofen and two to four weeks off the bike are the only way to heal properly. I am just recovering from this same injury. If you don't correct this and allow it to heal it will eventually cause real knee injury from you rknee not tracking properly. The hip, outer leg area laterally and outer knee area is all held in place by the IT Band. I'd almost bet money that's where the problem lies.
If you think this describes you inury... DO NOT TRY TO RIDE THROUGH IT! If you keep trying to ride through it... the pain will get worse and keep you off the bike entirely. Good Luck! Hope this helps.
My latest bout started differently than the one I had years back also, but my body/muscle structure is different now also (and I'm older). As fit as I am the ITB problems still flared up. And in hind sight... I really wasn't paying very close attention to properly stretching those areas pre and post ride, so it snuck up on me. Hope you get through it.