The most likely explanation is that you have had some shrinkage of the vitreous of the eye, and small crystals and folds have been created within its gel. This is called a Posterior Vitreous Detachment (PVD). Flashes can be caused by the vitreous pulling away from the retina where it has been attached and they may be apparent for some months when going between different lighting levels.
It is also possible in rare cases that the retina may detach from the back of the eye. This can be repaired but requires urgent attention and we would ask that you contact us if you suddenly notice a change of this sort, and we will provide you with an early appointment. The sudden arrival of a “curtain” or a grey shadow which is in a constant position despite eye movement will be prioritised.
It will be necessary to dilate your pupils at this time so that your peripheral retina can be observed, and you may find that your vision is unacceptable for driving for a period of approximately two hours after the examination.