What Causes Itching In The Legs When Under Stress?

Hello,

When ever I start to get stressed out or worried about something my legs become really Itchy and i scratch them until they bleed and still feel the need to keep scratching. the sting and hurt but i still need to keep scratching and once i start i cant stop. its only on my legs. is it a mental thing or am i allergic or something? please help?

thanks.