Flashbacks can be caused by any traumatic experience. Hallucinating can be very traumatic. So can going into labor. Or being mugged. Or being in a war.
Once I was in a really freaky car crash and almost died 'cuz my friend fell asleep at the wheel and hit a brick wall. I had flashbacks for about a month. I'd be in the middle of talking to somebody, and then suddenly I'd see a brick wall flying at my face, and then I'd snap out of it and realize I was just staring into space and had stopped talking to somebody in the middle of a sentence. I was completely sober during the accident and during the flashbacks. I thought the flashbacks were scarier than the accident was, because I was afraid I'd have them forever.