Thanks for your query.
don`t occur immediately after intercourse,especially when you are on birth control pills ,so it is not due to pregnancy .
It can be due to cold
or some infection if it was an unprotected sex.
It is also not due to birth control pills.
If periods are delayed only than think of pregnancy.
‘Hope I have answered your query, I will be available to answer your follow up queries, “Wish you Good Health and trouble free speedy recovery”