They think one of the reasons for O pain is one ovary has more blood flow to it than the other, causing the growing follicle to push painfully against the walls of the ovary. This is also why there would most likely be a little spotting at the same time, because the bursting follicle also bruises blood vessels. Since this is almost always in ONE ovary and not the other, and since we don't ovulate from the same ovary every single time, it stands to reason that you wouldn't feel O pain on the months you ovulate from the other side. I think that same article said that 1 out of 4 cycles will be painful in these women.... although I think they're being too safe in this estimate. I would say atleast half if not more of my cycles are painful.... I normally O from the same painful side for several months in a row and then have a couple non painful months.