Lots of things can affect your periods: loss of weight, weight gain, stress, lack of sleep, environment, other women's pheromones (a chemical all animals secrete - that's why women who live/work together will eventually all have the same periods), etc etc etc
If you're truly a virgin and have never had any kind of sexual contact, no, I wouldn't be concerned. If you HAVE had sexual contact, even if it was just mutual masturbation where semen could have had contact with your vagina then it is minutely possible you could be pregnant but I wouldn't even worry about that until you're at least a couple weeks late.
Most likely, your body is just adjusting itself and it will all even out in the end. Just be mindful of your schedule but don't set your world around it - especially when you're so young.
If you're truly worried about it and just can't stand the suspense, talk to your mom, aunt, school nurse or doctor and let them answer your questions.
Experience and mom of a 13 yr old