Are you still exersizing her? The sudden change in routine can cause stress which leads to weigtloss a small amout of excersize like walking her round the field (not ridden) could be enough to stimulate her, try new foods she could be bored, it could be someting more serious like colic or cancer so get a vet to look at her
Hey try to buy a round bale of hay. Try 6lbs of purina strategy a day that should get her weight up.
I would go back to your vet, or to another vet and ask about specific pre-natal care. You need a vet in this case not a dentist.The in-utero foal takes a ton out of the mare. You may be surprised that the foal will come out looking fine, while the mare is emaciated. The other thing to consider is that if the mare does not have adequate nutrition, she will run the risk of not producing milk, then the foal will not be able to nurse and get adequate nutrition.
I think you have the wrong community take a look at the large animal forum ..hope your horse gets better soon, What doe the Vet say about the weight loss, maybe once the foal leaves she will put on weight. Ask the experts about it and good luck