Jamie fell backwards and bumped her head. She briefly 'saw stars'. This is because her fall stimulated her:

Jamie's fall has probably stimulated the occipital lobe of the cerebral cortex.

This part of the part of the brain has a role in processing visual information that comes from the eyes, so stimulation of this brain region without the signal from the optic nerve produces a visual sensation without any recognizable shape that can be described as ''seeing stars''.