Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern...
Withdrawal method is not a good
contraceptive method to employ.It has failure rate of 20% in first year, which is very high.
Often there are secretion from cowper gland ( also known as pre-ejaculation) prior to ejaculation. This secretion can also contain sperms and can cause
pregnancy.
There is simple way to know if a woman may be pregnant, if she had a menstrual period in the time after having sex.
1. If yes, she need not to worry and is not pregnant
2. If not, she might be pregnant.
As you mentioned that you had regular period after having sex, this means you are NOT PREGNANT
Don't worry
Hoping your query is solved