Random question

The last time I had sex was on the 25th of june which was also my ovulation day. I am supposed to start my period today but i havent started. I usually get cramps a few days before my period and thats how i know that my period will be here but i havent had any cramps. So today i was supposed to start my period but instead I woke up really nauseous. I dont want to eat much and i feel like throwing up but i wont. I am just very nauseous. What does this mean??