if your experiences are in any way 'incapacitating' or worrisome, i'd suggest
making that doctors visit. if it seems more of a curiosity, then physicians are
unlikely to be of much assistance i reckon.
a practical explanation might involve "afterimages":
https://en.wikipedia.org/wiki/Afterimage
are the flashes always during times of activity? i'm unsure of what would
cause it, but i'm not recalling having experienced that if i was just sitting
around.