Why Are My Menses Delayed?

Hi doc, I am 24 yrs old gal, just need some help that I had a sex with my bf without any protection but he s sure that he had removed before his sperm comes out.. but I miss my period by 3days. I am nor getting why? Also did pregnancy test conceive test n result is negative... so any other reason? For this symptom? Pls reply soon Thanks.