Hello, Unprotected sex 3 days prior to period due date is very much safe if your previous cycle was regular. You also took emergency pill for extra protection against unwanted pregnancy. So, in this case, chance of pregnancy is almost nil. Period is late from expected date due to hormonal imbalance associated with taking the pill. Irregular diet habit, psychological factor, mental upset, weight change can cause less flow of menstrual bleeding. Here, you must take balanced diet with iron supplements, avoid stress, drink plenty water/ fluid, do physical exercise, eat carotene based vegetables and take sound sleep. Be well & always practice safe sex.