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.
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.
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.