I've been trying and trying to remember, because I do remember having a similar pain! but I can't recall exactly what it meant or what caused it. I think it happened when i was undergoing chiropractic adjustments, and everything was shifting. Sorry I don't have anything definitive for you.
I agree with what Pinkcat said, we do tend to get mysterious aches and pains as we cleanse, most definitely. Some of them are linked to very old illnesses and injuries that are resurfacing for a final release of stored toxins, be they emotional or physical. So try perusing your own past for any issues that involved the pelvic area. Could be you're releasing it.