It's really common for kids your age to have irregular periods for several years. As annoying as it is (cause it makes it harder to plan for when to be prepared for it), it isn't in any way concerning so you shouldn't worry about it. It's totally normal and to be expected. In fact it's less normal to have a totally regular period that comes at the same time every month at your age. That's not common at all.
Hello ! Don't worry, as you're quite young your period might not be very regular meaning that your period might be a few days late or early.
If you're still worried you can try seeing a doctor but I'm sure you're fine, just wait it out.