Thanks for the query.
There is possibility she might got post-coital injury
at the time of intercourse.
The wound might have closed by the blood clot. After displacement of the clot the bleeding might have shown.
So let her once consult gynecologist and get examined to find out the tear or any other possible cause for her problem and take treatment accordingly.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429