Missed period

Ok so I’m about to get into details here and sorry if it’s weird. 8 weeks ago I had unprotected sex with my boyfriend (he pulled out) and this was 3 days before ovulation then I had my period but it was shorter and lighter (5 days and only changed my pad once a day). Now it’s week 8 and I am 5 days late for my period. Why would I be late? I don’t think I’m pregnant cause I used these test strips by mommed and they’ve come back negative.