Thanks for your question.
Having sex is not a reason for irregular or missed periods unless a woman conceives.
If a recent blood pregnancy
test & pelvic ultrasound
is negative for pregnancy then naturally this is not the cause of her period problem.
She should see a Gynecologist & have evaluation for the cause of this period problem.
In women with irregular periods
, only an early ultrasound ( done during the first 12 weeks ) is a way to find out time duration of conception. Duphaston
is a hormone which may be used to induce periods, but, I think she should have investigation first, to find out the cause of her missed periods, as, she may need other medicine for a successful pregnancy.
Best of luck.