There are many possibilities. Generally this is because blood is leaking, and the reasons for this can be complex. It could be an issue for a hematologist or for a vasular specialist.
It could be from a simple vitamin K deficiency, hindering clot formation.
This is an indication for a complete work-up from an M.D.
If blood is leaking in the legs there is the possibility it may leak somewhere else, such as the brain.
You could be deficient in the vitamin called "bio-flavonoids" these are relatives to Vitamin C and they are very important in helping to keep from bruising. Bruising is due to broken blood vessels, and the bio-flavonoids keep them strong.
If they get worse or start to hurt, then I would sure see a doctor.
blood vessel could have broken, you could have bumped in to something without knowing-people do this all the time. If you are experiencing this alot, you should see a doctor.