Why do my feet constantly hurt even when I sit down?

Swelling. It would help with more information but assuming this is pain in feet while standing for long periods of the day and when you sit they still hurt suggests the possibilty of some venous insufficiency that uually is good in the morning and at the end of the day the pain is there. If you check your legs for swelling and elevate the legs , pain should be better. Please be evaluated by a physician.