What can cause a dog to not want to eat and be lethargic?

I have a 11 year old Chihuahua for 3 weeks now that has been not wanting to eat and just lays around the house all day. She does get up though to welcome us home and to bark at people that come by the house. She has been on the bland diet for the time she has been sick however I haven't really seen her poop much, she does pee. She drinks water but not a huge lot. The day this started she threw up her food and it was like a tube shape with mucus. She has been to the vet twice and the blood test and xrays were clear, she had some gas and noise in her stomach. The vet gave her a nausea shot and steroids and 3 hours later, she perks up and act super hungry. After 6 days from the shot, she quits eating again. I took her back and they gave her the shots again, and she ate but 6 days later she stopped. She is now on steroid pills but her appetite isn't super strong. Months before this she had a cough from time to time when she would howl or bark at someone but it has gone away. She has lost some weight also. The vet says everything came back clear and her lungs and heart sound really good. Last year she was spayed and every winter before that she would be clingy and whiny but now she just doesn't want to eat.