At 15 daughter never had period yet, afraid it might be primary amenorrhea?

Time for workup. Any girl that has not had her first menses by 15 should be evaluated for delayed development. Your primary doc could begin the process with some tests but will likely send you to pediatric endocrinologist to sort out the issue.
Could be. Have her seen by a competent md to rule out causes such as athletic amenorrhea or other forms of anorexia. A few simple blood tests and Progesterone challenge test will rapidly give you an answer.