Tatiana, Hoe did your doctor determine your due date? If it was based on your LMP, it may be somewhat inaccurate. If it was based on a scan between week 8 and week 19 it should be very accurate.
According to your due date, you ovulated around Sept 22. So sex from about Sept 17 to Sept 23 could have gotten you pregnant.
The fact that you are even asking this question shows that you are not sure that the other guy pulled out in time or correctly. It is always good for woman to question the effectiveness of pull-out. It interrupts a man's orgasm and is very difficult for him to control, unless he is very careful, mature and experienced.
If you have been trying to get pregnant with your boyfriend for a while, it will also be very suspicious that you got pregnant the very first time you cheated on him.
This is the best I can do with the limited information you gave.
Take care.