Did you have full-legal custody of her when you became a US citizen?
Talk to an immigration lawyer.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
It appears that she was already 18 (even if she was a LPR at that time) when you became a U.S. citizen so she must file her own naturalization application upon being a LPR for at least 4 years and 9 months.
If she was residing in the US as a green card holder at the time of your naturalization, she should have acquired citizenship as of that date. Consult with an immigration attorney to obtain proper documentation of her status.
Please note that this posting does not constitute legal advice or create any attorney-client relationship with the inquirer. Avvo is a useful forum to obtain general information on legal issues, but is not a substitute for direct, confidential consultation with an attorney in any legal matter.