There's an ingredient in some custard flavors that tastes, to some people, in higher concentrations, like vomit. At what percentage this occurs, or if it even occurs at all, depends on the individual. Some people just can't vape that ingredient, and some never have a problem with it. If that sounds like what you're experiencing, you might try diluting the liquid with some unflavored PG/VG and see if it helps. With or without nicotine in it. You can get a bottle of Humco brand glycerine (pure VG) from the skin-care aisle at Walmart (or find a similar product in most drug/department stores) for just a few bucks; this will reduce the nicotine strength of your liquid, but it can give you a rough idea of whether it is something you'd want to explore further, or not.