I have a what I think might be a bruise on my left leg the size of a quarter. And it has small purple spider veins around it, but hard to see? Why?

Two possibilities. It is common to find a bruise on the leg and yet not remember the injury that caused it. This becomes more common with age and with the use of Aspirin and other anticoagulants. If the bruise persists, it may not be a bruise but a cluster of dilated capillaries known as telangectasias. These are caused by backflow through an incompetant deep vein and are treated by laser or sclerotherapy.
Hemorrhage. Sv can spontaneously rupture beneath the skin resulting in a stinging sensation, then subsequent bruising is noted from the hemorrhage of the small vein. Local trauma may do the same thing. The sv become difficult to see because of the bruise, which is blood released for the ruptured vein escaping into the underlying tissue.
Bruise like vein . Brises on leg looks like vein get ultrasound of leg looks like vein.