I'm not in the medical field, but I've spilled my share of liquids (mostly coffee, mostly on new carpeting). "A lot" is very subjective, so lets talk fluid ounces.
I just did an experiment: I didn't have any blood handy, but I poured three fluid ounces of water on my kitchen floor to see what that amount of liquid looked like spread out.
There was more liquid on my floor than what appears to be on Joan's.
I'd estimate that there were around two fluid ounces of blood on Joan's floor, so maybe there were two-and-a-half to three fluid ounces present at the scene when you factor in the amount of blood smeared on the wall and the car. To me, that's a lot; I don't think I've ever lost that much blood from a cut, and I've had several cuts that required stitches. (I've donated much more than that at once; a typical donation is a pint, or sixteen fluid ounces.)
We don't know how much blood might have been absorbed by Joan's clothing or by any rag she might have used to stanch the flow of blood.
No witnesses stated that Joan had a bloody face, so we can probably rule out a nosebleed. Considering the reports of a woman with bloody legs, gynecological bleeding seems likely, but intestinal bleeding is also possible, as is an abdominal wound or a leg wound.
I still vote for a ruptured fallopian tube from an ectopic pregnancy, but I'm far from certain.