There are three possibilities for her bleeding.
1. she may starting her period a little bit earlier.
2. she have become pregnant. After conception, sometimes women experience a small amount of bleeding after implantation
3. She had a small amount of trauma in the setting of sexual intercourse that is resulting in a small amount of bleeding now. this may be due to the breaking of small blood vessels at the end of the cervix
. These blood vessels are quite fragile and often pop during intercourse.
However it is better to consult the OB/GYN.
She may examine her further and probing the history about the periods to identify the cause.
Hope this helps.
Please write back for further queries.
Wishing you good health.