Okay, I don't know when you started your period.
Some girls start their period around the age of 16 (which would be considered late), and since you are not sexually active, I'm assuming it may be because you started your period late and your cycle is irregular due to the fact you have not had your period for that long. (Which is completely normal)
On the other note, it can't be because you're pregnant. As you said, you're not sexually active. As you mentioned, it may be because of stress, as stress can minimize your period. Either that, OR you may have amenorrhea (lack of menstrual flow).
See your doctor about this.
i started my period when i was in 6th grade so about 12 i think and it was usually on time it wasn't till i started college that this started happening.. and yes my period is still continuing starting the fourth week and it doesn't seem to be on its way to stoping