Back of the knee pain is difficult to assess, even on someone that comes to see you in person. So any diagnoses I post here are purely speculation.
Baker's cyst or strain of the popliteus - maybe but doesn't really make sense that the pain gets worse with sitting.
Restricted blood flow - maybe, but again doesn't make a lot of sense since given the history you gave us.
So my best bet would be referred pain from somewhere else. I'd guess the back. A lot of back problems get worse with sitting and it would make sense that you could have strained your back while clearing trails. You don't have to have local back pain to get referred pain to the leg that is coming from the back. Also, you mention that the pain does not feel like a pulled muscle. Usually referred pain like I'm talking about feels like a dull toothache or a sharp shooting pain. Does the pain get distinctly better when you walk? Does it get better when you lie on your stomach. If these are true, you're almost certainly dealing with referred pain from the back.
So I would second the trip to the Doc or PT to get properly diagnosed. Especially if it persists for longer than 5 days.
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