Hallow Dear.
Withdrawal method is one of the most unsafe and unreliable method of
contraception. Before the ejaculation of semen, some amount of pre-ejaculatory secretions are expelled out which do contain some sperms which are capable of causing pregnancy.
It is not clear from your history whether the bleeding you had after the intercourse was at the due time of
menstruation or at some time. If it was at the time of menstruation, you do not have to worry about the pregnancy as missing a period is the first cardinal symptom of pregnancy; and moreover just before the menstruation, there is no egg present to get fertilized.
If this bleeding was apart from your menses, you will have to find out the evidence for any pregnancy. Pregnancy test on the overnight first morning urine sample about a week after missing period will help you to detect the pregnancy. Alternatively, you may opt for
Beta hCG test on your blood 10 days after the last
unprotected intercourse. Values of hCG under 5 mIU/ml denote 'no pregnancy' while values more than 25 mIU are diagnostic of pregnancy.
Pain in abdomen is not a symptom of pregnancy. Brown
bleeding during pregnancy is indicative of danger to the baby; so it is also not a sign of pregnancy.
I hope this helps you.
Dr. Nishikant Shrotri