B.,
Along with health issues and weight concerns, stress can cause a period to be very irregular, as can illness. Plus, when I was 13-15, I only got my period two or three times a year. It was just the way it was cause I was so young. Just FYI, I am a counselor who works with teens(actually all ages) who have expereinced sexual violence. If it is something that she is interested in, it is free if she lives in Washington County, and within certain boundaries(She needs meds, she's suicidal,etc), her parents don't have to know(though we prefer if they do)###-###-#### reaches me directly) We also have a free 24/7 hotline, 1800-###-###-####. Feel free to pass it on. Hope she's healthy and all works out well.
ALSO, wanted to go back and add. If she is trying to get pregnant, she might refuse to go onthe pill. Doing some life skills, either with you or with a counselor might help that mentality!