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It's definitely normal to sometimes start your period a few days early or late. While some people are lucky enough to get their period at the same time each month, there are tons of people out there who get their period within a certain time frame. Like Butterscotch. said, there are a ton of factors that can affect someone's cycle too.

I used to get my period every two weeks, and that was a more of an issue. Keep track of how often you get your period and exactly how early or late you get it. If it falls into Planned Parenthood said the normal cycle is about every 25-30 days, but it can be shorter or longer. If your cycles fall out of that range often it may be worth talking to a doctor, but if you're in range you should be okay as long as you don't have any other symptoms!