There are different types of speed cameras (some of which don't flash at all) but a conventional Gatso camera produces a rapid double flash. The purpose is to take two photographs, at a fixed time apart, from which the vehicle's speed can be calculated (by reference to the lines on the road). I've sometimes seen apparent flashes in my mirrors, only to realise that I've probably actually seen the sun glinting through the trees (or reflected off the surface of a following vehicle). I doubt that you've got anything to worry about.
Chris