Understanding souce and treatment for pain behind my left knee

I take Bikram yoga and a couple of poses recently have made me aware of pain in the posterior left knee. There's no pain with hamstring stretching or calf stretching. I feel it tweak in a pose were we are in the "W" position of the legs and lay our spine supine (Fixed Firm Pose: Supta Vajrasana). Also pain with toe stand (Padangustasana). I'm leary of that much compression right now and it doesn't feel great. Any ideas? I've been avoiding those poses during yoga. It doesn't feel inflamed and is not painful outside of that. No pain during dance class or when I teach Pilates or work.

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