Agree with the others, it may be a result of a rapid BG drop but it may equally be a migraine.
Mine used to start with a feeling of apprehension, pretty pastel-coloured flickering around the edges of my visual field and sometimes a feeling that sounds were actually occurring on the surface of my eardrum rather than out in the world. THEN someone would clamp a steel band round my head and start tightening. Any of that sound familiar at all?
Curiously I *never* had ordinary headaches and these were rare, which I thought was a pretty good trade-off, and even more curiously since controlling my BG I haven't had a one.