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There's always a chance of getting pregnant even when on birth control. But if you have taken it as directed, the risks are rather low.
You could still be having symptoms of the flu or even digestive symptoms. It's best not to worry yourself about it until your period is suppose to come. If you want to ease your mind, a pregnancy test 14 days after unprotected sex should be accurate or once you have actually missed your period.
I am sure you are just fine and experiencing the flu. Try taking a home pregnancy test in the next several days to know for sure.
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