Sudden Visual Disturbance and headache.

I see flashes of texts I am currently studying for my exam. The text moves from left to right in front of my vision. And my brain suddenly gets filled with the texts I'm currently studying for my exams. Last month when this happened I couldn't speak and was seeing the flashes of texts. It happened for like 2 minutes straight. I could see, hear but couldn't speak. After that I started to have a left sided headache which lasted for like 4 hours. I went to a local doctor and he suggested I get a cbc, thyroid and vitamin d check. My vitamin d levels were very low and everything else was fine. The doctor prescribed vitamin d supplements which I'm currently taking. Today suddenly I saw flashes of texts and the word read 'imperialism'. Why is this happening?