Posterior Knee Pain- especially when walking

[Bilbi0701]

Hello,
for about a month or so I've been experiencing posterior knee pain, more medially than laterally. It hurts most when walking (when i straighten the leg out to take the next step with the achy leg- making it difficult to walk normally). Walking up stairs does not cause pain, nor does it hurt when i sit/rest, and actually seems to relieve the pain when bending the knee.
It is not tender to touch, and i don't think it's swollen or warmer than the other knee.
I did not injure my knee recently or anything like that recently, not that i'm aware of anyway.
I've had bad knees since I was 10 that mostly made it painful to walk up/down stairs and do any exercise that put a lot of weight and pressure on the knee, but I don't think this issue has anything to do with that.. just giving some knee issue history.
Thank you for any help