Bruising in absence of any injury
is generally caused due to following reasons.
1.. Use of certain medications like aspirin
,pain killers,anti coagulants like warfarin
2.. Due to lack of platelets which may be caused by many types of diseases like leukaemia
3...due to deficiencies of clotting factor in blood or in Samos chronic liver diseases.
4.. As you age capillaries become fragile and skin becomes thinner.
I would advise you to consult a doctor and get all the relevant investigations done for proper diagnosis.