Hello, I can understand your concern for unexplained bruise on lower leg. You have not provide with your age, sex and related medical history of any medications or illness. In general bruise are common to occur and most of the cases it is due to simple normal physiological process. It occurs due to extravasation of blood under the skin. I suggest you to go for bleeding time, clotting time and platelet count. If these tests are normal then there is no need to worry about anything. I suggest my patients with recurrent bruise to take lot of vitamin C containing diet and all sour fruits. It helps in strengthening the blood vessels and therefore to prevent bruises. All the best.