Hello,
Withdrawal method is not full proof and sometimes pregnancy may occur. However Pregnancy occurs only during the time of ovulation. Ovulation is the release of a single, mature egg from a follicle that developed in the ovary. It usually occurs regularly, around day 14 of a 28-day menstrual cycle. Having unprotected intercourse on last day of your periods reduces the chances of pregnancy as the innermost lining of the uterus has been sloughed off during menstrual cycle.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Honestly... if you have unprotected sex yes it is possible