Can it be normal for severe, sprained ankle bruising to get worse after 8 days?

Old bleeding. Many times the initial bleeding is underneath the skin and in the deeper tissue. As this blood migrates through the tissue it may show up on the skin days after the initial injury. This does not necessarily represent continued bleeding. This should fade with another week or so.
Yes. It is important to be checked by a doctor. .Having a negative xray for a fracture does not rule out a tear in the ligaments or tendons. Bruising is a indication that bleeding has occured.