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This is most probably the initial stages of a condition called varicose veins
. It is caused due to incompetency of leg vein valves. The condition is exaggerated by long durations of standing, and subsides on rest or limb elevation. Consult a surgeon in this regard. Color Doppler tests of the leg veins may have to be done, to locate the site of incompetence. Limb elevation during sleep and wearing elastic stalkings would help.