Basically for the first 15 minutes of a bike ride it feels like it’s going to buckle under me if I get out of the saddle.
Had exactly this for some years, although when walking, after a hard low-speed impact involving my left knee and the Tarmac. After several visits to my doctor, who just wiggled my knee around, shrugged and told me it’s what I should expect, I’m getting older, I got another doc to have a look, he did a more in-depth examination, sent me for an X-ray for confirmation, which showed osteoarthritis in two places in my knee.
The last week or so it’s been very painful indeed, the Zapain codein tablets I’ve been prescribed have been in regular use, along with ibuprofen and Voltarol.
That looks like the OP’s future to me, but he really should get a sympathetic doctor to look properly at it, and get an X-Ray to confirm it