My healthy 1 yr old Siamese gave birth to 4 kittens 9 weeks ago. She is really emaciated. Help?

She has always been slim with little appetite. I fed her kitten food throughout pregnancy and since but she just doesn't eat enough. Have wormed her and taken to vets. They say to separate her from kittens to stop her feeding them. I try to but she pines and doesn't eat. Kittens are healthy boisterous and a good weight.