Thanks for the advice. Joanna, your experience sounds very similar to ours, though Willow is spayed. Still, perhaps there is swelling for some other reason?
Willow did better last night. Slept til 6, which is about normal. She is still straining, though. I've not seen her have a bowel movement in days, but I suspect I have missed something. She's still skittish about people being near when she goes to the bathroom, so if you get close she stops. (Did this when healthy, too). I suspect I missed something because she continues to eat and drink normally without any vomiting. So it HAS to be going somewhere! Still, she should not be straining like she is.
Anyway, she is going to the vet today. Hopefully we'll have an answer and a relatively easy way to treat the problem. I'll let you all know!
Cedar (tri), Willow (blen), & Holly (ruby)