thank you for your query.
In your case of spontaneous bruising,it can be due to usage of medications,ageing,sun exposure
Pain relievers such as ibuprofen, naproxen and aspirin interfere with blood clotting and cause bleeding into the skin tissue.also cortisone
injections,warfarin etc have same effect.
also associated swelling can be due to Varicose veins
and Connective tissue
disorders such as Ehlers-Danlos Syndrome
may cause relatively easy or spontaneous bruising.
this can be prevented by minor exercises.also get a blood work done by your GP.
wishing you good health