What Causes Delayed Menstruation Cycle?

hiya i had sex with my boyfriend on 23rd july on a friday morning at about 3am and it was too late for me to go get the pill. so we have both left it and had arranged to do a pregnancy test a few days after if i miss a period but i have about 2-3 weeks left till im due on next but im eating alot of fatty foods lately and my wee has started to go dark yellow. what could it be?