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Sex 3 days before a period is generally very safe because ovulation is over and a pregnancy is extremely unlikely, especially if you have an irregular cycle. Emergency contraception often changes the menstrual cycle, causing it to start early or even several weeks late.
If your period is late, it does not mean you are pregnant. To ease your mind you can take a test in about 10 days, if you don't get your period first. But your risk is less than 5%.