I had my last period on September 4 and my cycle length is usually 30 days so my next period should be last October 4 based on the calculations made in YourDays.com but I didn't get my period and I had protected sex with my boyfriend on October 9. Up until now, I don't have my period. I took a pregnancy test and it was negative. I also don't have any signs of pregnancy. What could be the cause of this delay?