Hey Sarah,
Don't be worried about this at all! You just started your period, and yeah they're kind of hard to handle at first! I first had mine at 14 then didn't get another for 3 months, then didn't get another for 4 months. Needless to say, everyone's a little bit irregular at first. Until your body figures out its rhythm and what's going on, you will be a little irregular. Sometimes women don't even have regular 28 day cycles, sometimes it is just irregular in its own. It's just an annoying thing we have to deal with! That's why it's good to carry around pads or tampons when you go out places in case you have a leak! Don't worry though, once you grow into it, you'll find cues so that you won't leak through your pants. I'm fairly regular myself now, though I do have times where I'm a few days or weeks early or late. It's just life unfortunately. There are ways of regulating it, but since you're 13 I wouldn't suggest going on the pill just yet!
Best of Luck! And don't hesitate to message me if you have any questions!