Why do i have bruises on my leg and don't know where it came from?

Clotting issues: Bruising can be result of simple mechanical pressure on the affected area . However easily bruising raises question of coagulation problem. It may involve abnormalities of platelets , clotting factor deficiencies , combination of both like von willibrandt factor deficiency . I think you need to sort out how you get bruised .If it is spontaneous , you need to see your pcp and he might refer you.