We had sex using condom 3 days before periods. Bt after 12 days peroids haven't cum. Whats the reason?

If that's the case. Its probably irregular ovulation. You had no earlier unprocessed sex? We never discourage a pregnancy test and I have to recommend you do one but sometimes people will not ovulate and the normal changes in hormones that create a cycle do not occur. If on the pill not unusual to miss one. This may self correct or you may need appt.