Houston, we have a sac. Just ONE, too, and my apologies to all you twin wishers. But there IS a gestational sac, measuring appropriately for my dates, and there's even a little speck visible in there- not anything recognizable or with a visible heartbeat yet, but it's not empty, and that's all one could hope for at this point in the game. The doctor said my corpus luteum looked good too, which is relieving since that's what produces progesterone until the placenta takes over around ten weeks.
My actual hcg level wasn't low at all, either (2191 to 3110,) it just wasn't raising as quickly as they like to see. But for now the doctor said not to worry too much, all looks well, and we'll do another ultrasound September 2nd. So. For now we are trying to relax.
Thanks for all your good wishes!