Thanks for the query.
pills can cause menstrual irregularities like break through bleeding as side effect.
If you miss the regular time that can lead to bleeding.
Usually this will be minimal and subside soon.
If the bleeding repeats or increases consult gynecologist once and get examined to rule out other possible causes of inter-menstrual bleeding
like cervical erosion, poly etc.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429