This used to happen to me when I was young, and I think it was usually when my weight was low. I had a problem with absorbing nutrition, so I was really low in weight.
The good news is that you aren't sexually active so don't have to worry about pregnancy. Whew, that's great because THAT is very nerve wrecking.
So, what could be going on? Well, hormonal imbalances often cause things like this. Producing too much estrogen or progesterone for example. Also, if a thyroid isn't working quiet right, that can cause hormonal issues. A slow thyroid is well known for this type of thing.
Any way you can visit your doctor? That's the best option. But know too that being irregular is fairly common at your age even if it is a 'new thing' for you at this time. good luck