Hi & Welcome.
Pregnancy should be ruled out first.
I can understand your concern.
If there is no bleeding at the time of sexual intercourse it does not mean that the membrane at the vaginal opening (
hymen) is intact and seminal fluid did not enter during the act.
Being physically active, especially in a sport like horseback riding or cycling or gymnastics, or fingering can cause the hymen to break.
Therefore rupture of the hymen should not be taken as a sign of virginity.
If the hymen was already ruptured because of physical activity, the bleeding and pain may not be felt at the time of first sexual act and there could have been deposition of seminal fluid and thus pregnancy can occur.
So, since she has delayed period it could be due to pregnancy,
hormonal imbalance,
stress or emotional upset, etc.
I would advise you to get the home pregnancy test to rule out pregnancy.
If it is negative, consult a gynecologist for proper evaluation of delayed period.
Hope the suggestion given above could be useful to you.
Take care.