Post Run Knee Pain

After a run (5miles+) my knees get really tight and stiff - it hurts! - until I jump in a hot shower. Any ideas on what this is and why it works like this? I’d think that if it was swelling, a hot shower would have a reverse effect. No?

Could be runners knee? Is the pain underneath the knee cap? I took me about a month to get over runners knee

I wish that simple. It’s just super stiffness in the knees. Almost like arthritis pain. And goes away with heat. Only flares up when running. And I’m 32. Have had it for years and years (have also done marathons for years and years.).