Welcome to the forum Heather, sorry to hear Eva is not well.
How old is Eva? I agree with much of what Wiebke has said, she's covered a lot of possibilities and advice there.
If the teeth are a problem, it is usually the molars - hidden behind the fleshy cheeks - that are the main issue. Incisor malocclusion tends to come about as a result of some form of molar overgrowth. Having her molars thoroughly checked would be a good first step. It is possible that there's something very slightly amiss in the back of her mouth - even something as simple as a small, sharp spur on one of the teeth can be enough to bother them.
Does it seem to be more of an inability to eat (picking food up then giving up after a bite or two), or does it seem to be a lack of appetite (simply not interested in food at all)? Is she eating as much hay as normal? The disinterest in veg is definitely not right, as you say she's normally very keen.
If the teeth are declared normal (by someone who really knows what piggie molars are meant to look like!), oral fungal problems should be considered. Oral fungal infections can present similar symptoms to dental issues - inability/reluctance to eat, dribbling - and it shouldn't be dismissed if there are no visible white patches in the mouth; these don't always occur with piggies.
Whereabouts are you? Perhaps we can recommend an experienced vet in your area, or even a rodentologist who is more likely to be able to do a dental examination and any necessary dental work without anaesthetic.