When i scratch my legs i get what looks like a hickey. it lasts for several days. what is the cause of this?

Tiny vessel damage: If it lasts for a few days and looks like a hickey, you are probably scratching hard enough to damage the tiny capillary blood vessels in the skin, and what you are seeing is a tiny bruise in the area you scratched. Do you have any other easy bleeding symptoms like nose bleeds, heavy periods, gums bleeding with brushing teeth - then may want to check if blood doesn't clot right.