I asked the vet -- hopefully -- if there was any chance that Cobweb was just losing weight because Lucy chases her around. The vet said no, there really was no chance that she'd lose this much weight for that reason. Shannon wanted me to ask her whether our way of feeding her might cause something like this (we leave out plenty of food all the time), but she said no to that, too. Her response to these questions was to say that she believes that there is something significantly wrong with Cobweb which is the underlying cause of the weight loss. That was very clear and to the point, and left me feeling even more worried. I kept hoping against hope that this wasn't anything serious, but now it looks like it probably is.
Tomorrow I'm calling the vet to schedule Cobweb for an ultrasound. At least it won't hurt her.