My thighs sometimes itching for a longer time and becomes red. what may be the reason?(i drink green tea once daily, is it the reason ?)

Chafing: Probably your thighs are rubbing together. Try liberal use of powder, and avoid rough material like denim.

Dermatographism: Dermatographism means the ability to write on the skin. People with this form of chronic urticaria are sensitive to touch and pressure, as well as scratching. The areas of skin that are touched or scratched get a raised, red and itchy rash. While many people with chronic urticaria have some features of dermatographism, people with true dermatographism have pressure as the only trigger.