what you are describing in medical terms is called ecchymosis
you need to be investigated for many medical conditions like liver
disease, cancer of the lymph glands.
or it could be a blunt injury
that you might not be aware of.
treatment usually involve ice application, elevation of the limb affected, rest,painkillers and heat application.
suggest you should visit a physician for further followup and treatment.
hope have helped to answer your query.
get well soon.