Any unprotected sex can result in pregnancy
. Withdrawal method known as "coitus interruptus" does avoid pregnancy. But the secretions prior to ejaculation also contain few sperms and can result in failure of this contraceptive
method and pregnancy though rare.
There are less chances of your last contact making you pregnant because your endometrium would not have been receptive considering the menstrual cycle phase.
You have mentioned that you have had many contacts prior to the last one. If these contacts were also unprotected, chances of pregnancy increase. To ensure efficacy of any method be it the withdrawal method, it is a must that the method is used in every contact. So if you have not followed withdrawal method in your previous contacts then you could be pregnant.
Since you have missed your periods, you should do a urine test for pregnancy. If positive, do consult a gynecologist and do not take any self medication
If negative, then the possibility of delayed periods can be kept and you can use medication to menstruate by consulting a gynecologist.