Maltese - Lethargic And Guarding His Belly.?

Symptoms that you are mentioning can be due to serious illness if continue for more than 24 hours. If so, then following can be reasons.

1. Anemia
2. cardiovascular diseases
3. Hormonal disorders
4. Infections
5. Electrolyte imbalance
6. Cancer
7. Disease of skin, eyes, or any other part of body
8. Immune diseases
9. Side effects of medicines
10. Toxic reactions

There can be many more conditions that can cause lethargy in dogs. If lethargic symptoms don't last for 24 hours and your dog become fit then no need to worry.