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

Depends: You should immediately go see your doctor. It can mean a number of things, some of which are dangerous and need medical attention.

Have it evaluated.: It may be inflammation or an infection. Have it evaluated by the surgeon.

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.