You should start counting from the first day of your last period. So if you had a perfect 28 day cycle, you would have expected your period to arrive on 28th April but unless you have reason to believe you could be pregnant, the fact that it hasn't isn't anything to worry about.
If you've just started your periods then you can accept them to be irregular. Many teens find that their periods come at different times each month and it's not uncommon to skip one altogether. Most people find that their periods regulate as they develop and finish puberty but some women are always irregular. You might find it useful to start counting the days of your periods, to see if you can work out a pattern of what is normal for you and when to expect your periods.
If you don't think you could be pregnant and your period is usually regular then it's unlikely that you have anything to worry about. Periods are funny things, they can be affected by all sorts of factors. Hormones, stress, routine changes and illness can all change your usual cycle.
If your period doesn't make an appearance in a week and there is a chance you could be pregnant then it's time to take a test. If you're really concerned then there are 'first response' tests which detect the pregnancy hormone one day after a missed period which you could take now. Standard tests usually require you to wait a week after you have missed a period in order to work correctly.
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