Confused 😱

Last month I had unprotected sex with my s/o the day before my AF. AF came and went as expected. No intercourse since, (we have since gone on a break). However, I am now 4 days late for this AF, when I am normally like clockwork. Is there a chance of implantation from last month as it was the day before my AF? I haven't been feeling too great, and am now wondering if there is a possibility of being pregnant! I feel silly asking as logic tells me no, as I've had a period since, but I have to ask! Could this potentially happen? xx