So, almost 1 yr post tkr and after walking, i have a lot of pain under the patella. what could this be? it stays this way for days.

A few possibilities: Depending on how your surgeon performed your replacement, several issues can cause anterior, kneecap pain. If your patella was left 'as is', and not resurfaced with plastic, it can still be a source of pain post surgery. If a plastic component was fixed to your patella, too thick a combination can result in pain. In either case, a knee whichdoesn't fully straighten can give you pain with walking.

Tendonitis possibly: If your patella was resurfaced, it could be tendinitis. If not, this could be from not resurfacing. Tendinitis can be due to weakness or tightness of you quad. Scar tissue can also cause this type of pain.