Hello,
Welcome to HCM,
The strongest possibility that comes to mind is a post coital
injury, particularly if you had rough, violent sex.It could be a vaginal laceration or tear that has healed itself and so bleeding decreased.
Other reasons are cervical polyps, cervical growths or erosions that bleed on intercourse.Endometrial polyps also present in this way.
If you were pre menstrual, vigorous sexual activity and resultant
uterine cramps could have resulted in an earlier period than usual.
Hormonal medications or pills could have altered your normal date of period.
It would help to know your dates of menstrual cycles, whether such an event has occurred prior after having sex, or whether you are on any medications.
Consulting your Gynecologist, detailed clinical examination and appropriate testing will establish a specific diagnosis.
Thank you.