When I stand up from a sitting position I can’t straighten my leg due to knee pain?

I feel acute pain when I straighten my left leg in an effort to stand, after sitting. I’m Ok in the morning, climbing stairs, walking etc. The pain is located on the inside part of the leg, at knee level. Joint pain, maybe? Nothing is swollen.
It takes maybe a minute before the pain leaves and I can stand up and walk. If I walk too much I , the condition becomes worse but not while I’m walking. It’s only the next time I straighten my leg after sitting that the pain is worse than if I hadn’t walked too much.

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Tags: joint pain, knee level