Missing Period...any female or doctors advice would be appreaciated?

I have been under a lot of stress at home and at school recently and i haven't had a period since november, i had lost a little weight but i have put it back on again, im taking multivitamins and i know i cannot be pregnant as my boyfriend and i have always been very carefulwhat else could this be?im very worried any advice would be greatly appreciatedIm 18 years oldif i said im not pregnant then im not pregnant!how many times to i have to tell you people I-AM-NOT-PREGNANT!!!Thank you to the people who understand that...

Have you taken a pregnancy test because you can't really be sure your not pregnant unless you've taken one