These all are symptoms that you are mentioning and many conditions can cause these symptoms. Your dog might be having stomach upset that is causing vomiting. This condition can cause loss of appetite and weakness. Your dog needs to be investigated for the following conditions.
1. Changes in diet
2. Eating of grass or garbage
3. Gastrointestinal obstruction
4. Intestinal parasites
5. Intestinal and gastric tumors
6. Gastritis
7. Excess gastric acidity
8. Overeating
9. Eating rapidly
10. Exercise after taking meals
11. Food allergies
So, you should take your dog to vet for appropriate requirements.
1. Changes in diet
2. Eating of grass or garbage
3. Gastrointestinal obstruction
4. Intestinal parasites
5. Intestinal and gastric tumors
6. Gastritis
7. Excess gastric acidity
8. Overeating
9. Eating rapidly
10. Exercise after taking meals
11. Food allergies
So, you should take your dog to vet for appropriate requirements.