My son is a little older (3 now) and has reflux. When we take him off his reflux medication (have done this twice now) he wakes early. Usually around 5am and refuses to go back to sleep. Start him on the losec again and he's fine. Maybe your little one has a bit of reflux/food intolerance if she only does it sometimes? Maybe try a food diary to see if you can figure out if there is any correlation?