Suddenly My Dog Can't Walk Well; She Walks A Bit And Then Promptly Sits Down .. Very Unlike Her ... Constipation, Can You Help?

You have not mentioned age of your dog. If he is aged then some serious medical conditions can cause loss of interest in walk. Constipation in dogs can be due to

1. Lack of fibers in the diet
2. Dehydration
3. Ingestion of foreign material
4. GI obstruction
5. Hernias in the perineum
6. Some bacterial infections
7. Enlargement of prostate in male dogs

Treatment depends upon underlying cause. So, take your dog to vet for proper diagnosis and treatment.