I hit my shin bone on a shopping cart 3 days ago and when i walk it hurts bad it has a purple bruise and it's always hurting what do i do?

Perhaps merely a localized post-traumatic hematoma in area of injury. Are you taking aspirin or any other medications which could affect bleeding? Cannot exclude a bone injury, but this would be rather unusual. Since you appear to emphasize your symptoms, suggest you contact your doctor. An X-ray of the shin area might clarify extent of problems.