Thanks for posting your query.
Bruises can happen due to-
1.) People platelet and blood coagulation disorders can bruise easily.
2.) Being deficiant in vitamin c
, your more prone to bruise easily,due to weaker blood vessels.
3.) Rare genetic disorder called von Willebrand disease
, which prevents the blood from clotting properly. This disorder affects at least 1% of the population.
4.) Bruises in elderly people may be due to thinning of the skin.
6.) Maybe you just hit something harder then you expected.
7.) Heavy and rigorous exercise may cause bruises, which results from microscopic tears in blood vessels underneath the skin.
Visit your doctor for further investigation and treatment plan according to it.