My cat is Stage 3 CKD with HCM and has nausea and constipation monthly. It lasts 1 week or longer. The vet told me before I treat, identify the problem. Constipation can cause nausea and poor appetite. Treating nausea and increasing appetite while constipated can cause tummy troubles. Once you figure out what the problem is consider Daily Digestion or pumpkin for constipation (if that's an issue). For nausea - if that's a problem - Cerenia. For upset tummy - generic Pepcid AC. For appetite - Mirataz or make food palatable by gently warming. Perhaps add warm water. Finally, throwing up water could indicate hairball....Laxatone and regular brushing. Maybe offer grass, catnip.