More likely your boyfriend's; it sounds like you were already pregnant when you slept with the guy a week ago.
Get an ultrasound about your fifth week from when you think you might have conceived, and see what they tell you about your estimated due date based only on the size of the embryo and its development. Tell them that your periods are very irregular and you can't judge when you ovulated by when your last period was. (Or else, just tell them you can't remember for sure when your last period was.) Don't let them get you to tell them a period date and then count it off on a little chart or wheel, ask for an estimated due date based on the embryo's measurements. Then you can plug the estimated due date from the baby's measurements into a conception calculator online, or just count manually back 266 days from the e.d.d. to reach an estimated conception date.
Thank you for your help x