Why is my period 9 day late? 15 years old.?

hiya. i am 15 years old and my period is 9 day late. i have never had sex so im obviously not pregnant. My period is usually early by about 2 days or late by 3. i dont understand my it is so late now. The only thing that i have changed about my diet is that im not eating chocolate and sweets in the week (whereas i used to eat that every day). im not stressed but i have had exams recently. before my period i get stomach pains that are quite bad for a bit. i have had a headache for the last 3 days and on tuesday, i did get stomach pains in my lower abdoman an i thought i would get it yesterday but i didnt. what do you guys think??