Although local injuries are most common cause for such Tyler of bruises but in absence of any history of injury other reasons can be....
Platelet disorder and co agulation disorders
Use of medications like aspirin
,non steroidal pain killers like ibuprofen
,anti coagulants like warfarin
use of certain products like fish oils,ginkgo,garlic etc.
Rarely it can be due to underlying medical condition like leukemia
You must consult your doctor and get investigated properly.
Treatment would depend upon the cause.