It almost sounds like you have a foot drop, which is nerve damage that most commonly occurs at the head of the fibula (the bone is at the outside of the knee).
If you cross your legs frequently, this can injure the nerve, leading to weakness of the muscles that help to flex the foot.
However, most people with true foot drop have to swing their leg outward when walking.
So, you don't quite fit that clinical picture.
Here is information about foot drop for you to read to see if it fits:http://www.mayoclinic.com/health/foot-drop/DS01031
A problem with your calf is also a consideration given the pain you have with flexion of the foot. A DVT is a possibility and for this reason I strongly recommend that you see your doctor for an evaluation. While you don't have any other of the classic symptoms (swelling of the calf, warmth or ?redness), I still feel that you need to be checked out.
Baker's cysts are usually pretty obvious as you can feel them in the back of the knee unless they have ruptured.
So, to summarize, I strongly urge you to see your doctor today given the severity of your symptoms--DVT and a nerve problem are what I feel need to be ruled out.
Feel free to ask any follow up questions and I will be happy to discuss further,
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