Small bruise on calf, tired heavy legs. ultrasound done and ruled out dvt. what causes this?

Many, many things: But you should be thankful there is no clot.

See below: A small calf bruise may be from a focal hematoma from trauma or from underlyin blood disorders. Us is heavily dependent on the technologist's skills and a normal study does not always truely rule to dvt, although DVT is more commonly associated with leg swelling and pain. A focal bruise may however be from a thrombosis of the superficial vein which may nor may not have been evaluated on a DVT us.