Congratulations on the big weight loss! That is a huge breakthrough :).
Only a doctor can diagnose you and if you're having this much pain it's critical that you see one. Pain that is 'burning' is usually associated with injury to the nerves. Sometimes burning pain down the thigh is caused by something called meralgia paresthetica which refers to compression of one of the big sensory nerves to your legs. I don't know if that can cause pain all down to your ankle or not. Some of the other signs and symptoms that can be associated with it are:
A burning sensation, tingling, or numbness in the same area
Occasionally, aching in the groin area or pain spreading across the buttocks
Usually only on one side of the body
Usually more sensitive to light touch than to firm pressure
You're going to have to see your doctor about this. He/she may press on that nerve to see if its causing your pain, take x-rays etcs. Good luck!
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