What Causes Lower Abdominal Pain While On Birth Control Pills?

Hi every 2 week or so I get brown watery discharge very light and then I bleed for 2 days or sometimes I don't its just brown watery discharge,I've had lower right abdominal pain for a couple o days tht has now gone away,I have been on birth control for nearly 8 years my recent was the implant tht was ok at first however made me bleed day in day out,however since the implant stopped workin at the three year mark I am so much better n I now go a week two weeks not bleeding or anything but then I get this discharge plz help