Thanks for the question.
This looks like bleed within the subcutaneous tissue
below the skin.In cases of unexplained bruises without any trauma one needs to investigate the coagulation status of the blood. Usually these kind of bruises occur due to subcutaneous tiny hemorrhages.There can be various causes which can normal coagulation of blood leading to such hemorrhages.I think you should see a physician to get clinical examination done.The doctor may initially want to check your platelet count
, bleeding and clotting time. Even medications can cause this.
One needs to investigate underlying cause to plan the appropriate treatment. This should not be left un-evaluated. Few patients might suffer from internal bleeds, which may have serious outcomes.