ShiPom Health Concerns

About 2 months ago my ShiPom dog who is 2yrs old started acting different by not wanting to go outside, not being able to hold her food down and loose bowls. I took her to the vet, he told me to put her on a chicken and rice diet and then slowly add it back. The vet couldn’t find any thing wrong with her. Some days she is active and some days it’s like all she wants to do is sleep. Has anyone else experienced this? Please let me know !!