Vein running over knee cap swells, painful and then bursts. This happens a couple times a year. Leaves large hematoma. Whats wrong?

Varicose veins? . I completely agree with dr. Landi. This is not something you want to ignore. You should see a vein specialist and have a standing venous ultrasound evaluation so you can get to the bottom of the problem and a treatment plan can be put together.
See Vein Specialist. From your description it sounds as if you have a varicose vein in this area or a blue(reticular) vein. The fact that your symptoms are recurrent and associated with bleeding and hematoma formation indicates that there is an underlying problem with that vein. You should see a vein specialist for a venous evaluation and possibly even localized treatment such as sclerotherapy or localized excision.