For about the past two weeks, I've been waking up to two phenomena. Either half of my room looks green, as if the sunlight from my window is green, or I see green polka-dots with a yellow border. It's almost like an afterimage. The polka-dots move when my eyes move, but the green tint is always the same part of my room. They've never happened together, but I have one almost every morning. I haven't had any changes in medication or diet. What could be causing it?