Thanks for your query at DoctorSpring.
You have experienced what is known as post coital bleeding ( bleeding/ spotting after coitus ).
You should consult a gynecologist to know the exact cause for this, as it should be evaluated, however I would tell you the possibilities.
It could be traumatic intercourse causing a small vaginal tear or laceration, or a small cervical tear.
There could be small lesions on the cervix such as a cervical polyp, cervical erosion, or a cervical growth, which could be responsible for the same.
Polyps protruding out of the uterine cavity ( endometrial polyps ) also can cause such bleeding.
You have not mentioned the dates of your periods, so that I could corelate it with the date of your abnormal bleed, otherwise I would have thought of some pregnancy related complications.
Only a complete evaluation ( per speculum and per vaginam examination ) by a Gynecologist, and appropriate testing by ultrasound, would reveal the exact diagnosis.
Take care, and feel free to discuss further.