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This pain is entirely down the back of the leg, like this:
buttock, into the back of the thigh, back of the knee, back of the calf to the foot?
Leaving the front of the thigh and shin spared?
Is this a similar pain on both legs?
When you have pain in those areas you describe, the thigh, the calf, the groin area -- in both legs -- and aggravated by stretching (e.g. crossing the legs) -- this seems to be related to nerve pains.
These nerves start out in the lumbar spine, and when stretched (as you are doing when crossing the legs) could be the cause for your pains. Different nerves innervate different areas of your spine, so the upper lumbar nerves could be responsible for the hip and groin area of pains, while the lower lumbar can be responsible for the thigh and calf.
So, the most helpful thing here would be to get an exam by your doctor. They would likely order a MRI of the lumbar spine for further evaluation to rule out things like "lumbar stenosis" or a herniated disc in your lower lumbar spine - which can cause symptoms as you describe.