Manda, I think you would know this and understand this better than almost anyone on the board. You know your daughter, and you know bulimia.
Trust what you know, and trust that your daughter is following in your footsteps.
Best wishes.
This is definitely a sign that your daughter is starting to develop a problem. Because of all the influence from the media, the internet, her friends (I am not sure what she is exposed to) she might have got a faint/vague idea that vomiting would make her thin. Because she does not know the dangers of this, it is especially dangerous to her health! I would sit her down and talk to her about it. If she denies it, seek help from a doctor or a therapist. However you go about it, she DEFINITELY needs help! Best wishes.