I called the nurse first thing this morning to inquire about the spotting. One of the first things she asked was if I'd had intercourse (which could have irritated the cervix) and of course I said no, since they told us not to until after the blood test. She said pretty much what I've read: it could be implantation spotting or it could be "leftover" blood draining out (I'm not sure if that means it would have been from irritation around the transfer time, but that's what I'm assuming). She told me to drink Gatorade for hydration and electrolytes, and that we'd know more tomorrow after the blood test. Oh, and to call again if the color changes or the flow increases or the cramping increases.
I'm really hoping it's implantation spotting. I had crazy dreams last night that all had this overlaying feeling of contentment, likely because I read about the possibility that it's implantation spotting about two seconds before I went to sleep.