There are many reasons why a woman can bleed, and so it is not possible for us to say for sure why this happened to your girlfriend.
During sex there is a chance that the lining of the vagina is damaged (Say by a fingernail when someone is being fingered). It is also possible that women can have small amounts of breakthrough bleeding even when they do not get their regular periods because of the pill.
If this is an ongoing issue, or if she is having any other symptoms it is a good idea for her to see a health care provider. They will be able to take a full health history from her and if needed do any exams and/or testing needed to figure out what may be the cause.
It is also important for all women to do regular Pap screening as part of their general health care.
Please leave a comment to let us know if this answers your question or if you need more information.
Health Nurse
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