Have you seen a doctor? Trying to figure our medical problems over the internet is not great: I am not a doctor, and I can't see you and work with you directly. A health care professional can work with you interactively and test out ideas, and work with you to come up with suggestions.
Given all that, I suspect you have some inflammation going on. Often in cases of inflammation, once you have "warmed up" the body, it moves relatively pain free, but once you cool down again, the pain reappears. Now the question becomes, why is that knee inflamed? It could be due to some small tear you have in the cartilage of your knee (worst case) or a slight strain to the ligaments there (best case.) Again, a doctor would be able to test you knee and maybe give you a better diagnosis.
What to do about it? Rest. Leave it alone for a while and let it heal. Don't do Child's Pose or any postures that involve a deep flexion of that knee. Try something to bring down the chronic inflammation there (like ice, elevation or
earthing.) Adopt a policy of "zero tolerance" to any pain there - pay attention to little tweaks and stop doing whatever tweaks that area. In time, it should heal nicely.
One final idea: you could try this
exercise for the knees which has helped many people. But again, pay attention, if this makes the pain worse ... don't do it!
I hope this helps
Cheers
Bernie