Although most important cause of bruising is trauma but there can be various other reasons for it like..
Platelet and clotting disorders.
Consumption of medicines like aspirin
,anti coagulants,pain killers etc.
Even fish oils,ginkgo,garlic etc can also cause in explained bruises.
Old age in itself can result in bruises because of fragile blood vessels.
Lastly some serious medical disorders like leukemia
can also cause bruises.
I would advise you to consult your doctor and get investigated properly so that a proper cause can be found.
Treatment would depend upon finding the cause.