Hi. 20g weight difference is nothing. There can be more of a weight difference just between full/empty bladder alone. However your girl has had an astronomical weight loss so there is definitely something going on. Was your vet able to do a full examination of the back teeth? (This can be difficult due to food being in the way) and is your vet particularly cavy savvy? A knowledgeable vet, whether an exotics specialist or just a 'normal' vet with a specific interest in piggies, are worth their weight in gold. If your vet isn't particularly knowledgeable I would seek out a more experienced vet.
Pretty much every health condition that piggies can get can cause weight loss so this alone would not be enough to even start diagnosing the problem. Are there any signs of pain? (General withdrawal, becoming quiet, sitting hunched up, fluffed up fur, squeaking etc). Are there any indicators of problems with the teeth? (Front teeth slanted, front teeth overgrown, reluctance to eat harder foods, excessive drinking, pulling faces while eating or flicking their face to the side when they move food around their mouth).
We have a vet locator in the top bar which might help you find a knowledgeable vet in your area who can start to look into what the problem may be