Thanks for the query.
This discharge can be possibly due to side effect of the birth control pills.
These pills can cause menstrual irregularities as side effects.
And inter-menstrual spotting is one of those side effects.
So if the spotting stops by it own, no need to worry.
As she is taking the pills regularly, the possibility of pregnancy
is very less.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429