Don't be surprised if your vet cannot tell her age either- if you don't have a snake from it's date of hatch (or birth), there's just no telling, because their growth depends on conditions- ie. plentiful food, + favorable temperatures & time to digest. For snakes that brumate or just fast in winter, remember that takes time away from growth also. Even with rattlesnakes- where they add a rattle with each shed, there's no way to tell how long each one took to "earn", because a shed is just a milestone of growth, not the time it took. All we can do is make "educated guesses" about the age of a snake.
Maybe she needs a larger water bowl? Remember that in a warm enclosure, much evaporates too.