Six weeks since finger amputation, the stump is suddenly swollen, reddish. Why is this?

Inflammation. Can be due to infection, nerve irritation, retained sutures, secondary trauma and sensitivity or if wrapping to reduce swelling was done improperly. Speak to your surgeon.
Have it evaluated. It may be inflammation or an infection. Have it evaluated by the surgeon.
Depends. You should immediately go see your doctor. It can mean a number of things, some of which are dangerous and need medical attention.