I wonder if its your pelvic joints relaxing. I get a pain around where I imagine the joint is it feels like pulling through the front of my pelvic floor. The physio explained that the bones are separating and that's pulling the muscles into position they don't like. Mine gets a little bit better if I brace through my pelvic floor and lower abdominal muscles. I wear a pelvic support belt when I'm at work, it definately helps, and icing it for about 10miniutes at night also helps, it does look a little funny though.
Hope it gets better.
Tracy