I've had memory loss before I started on my lithium. My therapist attributed it to my lack of sleep and how down I was (not sure how true that was, in retrospect).

On the lithium I am experiencing other fun side effects but not any (extra) memory loss. I'm on an entirely different cocktail than quite a few people on here though, so it can entirely be the meds doing it to you. I would read carefully through the side effects to see which one causes it, and have a nice conversation with your doc about it.