It can be different things aside from pregnancy. Stress (including the stress of knowing that you had unprotected sex), re-regulation of your cycle after coming off birth control, normal variables, etc.
If you don't get your period by the 15th, wait until the 25th and a do a home pregnancy test. Use the FIRST morning urine when you get up as this is the most concentrated in terms of pregnancy hormones.
In the future, make sure you refrain from unprotected sex. Buy an assortment of condoms and let him choose from the lot. Don't wait for the man to produce those condoms!
Hope this helps! Good luck.
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