What Causes Delay In Menstruation After Sexual Intercourse?

Hi. My bf rubbed his penis over my vagina, and he did ejaculate, but we are pretty sure that no insertion took place (nearly 5th November) I had my last period on 10th November, and this month (december) i had no periods. Today, i got my urine pregnancy test done, but the result was negative. I never had any problem of delayed Periods in past.
What should I do?