My placenta isn't a perfect disc shape - it has a comet tail that wraps around towards the back. It looks like the tail may have separated a bit, which led to some bleeding. That blood formed the giant clot, which passed through the cervix - taking with it the chorionic fluid.
Yep, there is also the lesser-considered second sac of fluid around the amniotic sac. The second sac has it's own thin layer of fluid. It's not something that usually ruptures, but it can happen. And it is just a theory - my amniotic fluid measurements are low, but they were never huge anyway. As long as the baby seems to be doing well we are going to just hope for the best.
Meanwhile, I am exhausted. A hospital is no place for sleep, what with the IV fluids being changed at all hours and the monitors going off for the tiniest reasons. I had to have the IV in the back of my hand since it was so freezing when we got there that all my other veins were shrunken. My fingers are now stiff and swollen and my hand is giant and sore. I can't really type with only one good hand, but I just wanted to post an update.
Tomorrow: why to pack a "just in case" bag, even if you think you have lots of time left...