I have a burning sensation in the back of thigh, aggravates when skin comes in contact with something or while sitting; sort of shifting burning.

Contact Dermatitis?: It is very difficult to tell what this might be without seeing it. I have seen people who are allergic to nickel metal and who have contracted this itchy skin condition by sitting on metal or cast-iron benches. One lady worked in an outdoor furniture shop recurrently had this dermatitis. In short, it's very essential to see a dermatologist to have proper assessment of this problem.