A flashing light means that your mind is playing tricks on you. Whatever you were thinking at that moment is untrue. Sometimes I used to smoke pot and all the lights in the house would flicker and everyone would get freaked out, so I would apologize and clear my mind. I don't smoke pot anymore because it gets in the way of spiritual enlightenment for me.
I used to get those too, it means the same thing. Although I only got the internal ones when I smoked weed, the external ones (real lights) I still get.