I have the same thing happening this time around, and quite frankly, I don't think it's round ligament pain. I've had round ligament pain - that feels totally different, more of a deeper pain than this is. And it's definitely not BH contractions, although if you get one while this cramping is happening, it can be excruciating! The closest thing I can compare this feeling to is a leg-cramp, or like the OP said, a charley-horse. It hits suddenly, it's obviously in the abdominal muscles (not the uterus - so it's nothing associated with uterine contraction or stretching), and you can hardly move until it goes away.
I get mine when I roll over in bed, and often when I get up from bed, or when I get down on the floor to pick something up. And with sex - whether I have an O or not.
My guess is just abdominal muscle cramps, similar to what's been theatening to happen in my legs for weeks now. But if anyone else knows what this is, I'd love to hear it!