I did the math (or rather my spreadsheet did), and Penny lost 9% of her weight between 11/27 and 11/30. She still looks the same size, but that might just be because her fur is sticking out. The silver lining is that, since Penny was very overweight before all this, she could afford the weight loss. I'm not so worried about whether she'll gain the weight back; I just want her to stop losing weight.
The vet appointment is scheduled for about 5.5 hours from the time of writing this, so I'll post an update sometime after that.
The vet said it's GI dysmotility; Penny's back on the Cisapride and will remain on it indefinitely. The vet said that any number of things can cause GI dysmotility in older pigs. He managed to draw blood without anesthesia (he drew it from her leg), and according to the results of the first set of tests Penny's liver and kidneys and everything are functioning normally. I should get a call about her red and white blood cell counts and things like that by early next week.
Penny is currently eating her pellets, which is a relief.
On 12/1 I forgot about weighing her until after she'd had her veggies, but she was weighed at the vet when she'd only had a little bit of lettuce (for hydration on the way to the vet), and her weight was stable compared to 11/30. Yesterday (12/2) when I weighed her, she'd gained 62.9 g from 11/30.
Yesterday I got an email from the vet who did Penny's well check in October (he asked me to keep him updated on Penny's condition back when we started her on the meloxicam), and he said Penny's weight loss could be caused by stopping the Cisapride but that he's usually careful about putting pigs on Cisapride long-term because it can cause electrolyte imbalances. He also told me which vet to ask about it if I brought Penny in for an appointment, and it was the vet who saw her on 12/1. That vet didn't mention anything about electrolyte imbalances and said it was safe to put guinea pigs on Cisapride long-term, so now I'm a little confused. The vet's office just called a few minutes ago to check in and see if I had any questions, so I asked about Cisapride and electrolyte imbalances. The office will call me back when they have an answer for me.
Glad to hear she's eating better now!
Do make sure any exam includes an examination of her teeth in case she is losing weight. It's helpful to keep the details of this page (signs of malocclusion) in mind:
If Penny is eating all her other foods normally, does the fact that she's slowed down on pellets still point to a dental issue? I was thinking it was because she ate most of her 12/1 pellets in the evening and so she might not be as hungry earlier in the day, since most of her pellets seem to disappear in the evening.