Hi Pearshaker,

Sorry to hear you are in so much pain!
When you are on CD16 it sure could be ovulation. ARe you taking the pill regularly? I mean same hour every day etc? I guess you can still ovulate when on the pill as various women do get pregnant whilst on the pill.
It can really be quite painfull (my O-pain is pretty painfull too) and can last 2-3 days in my experience. Why not get an OPK and see if it is possitive? That way you know if it is ovulation.

Good luck, hope the pain subsides soon!