Has anyone had any experience with diarreha or loose stools only in the morning?

Tatum is 5 mths old and eat Natures Variety Instinct as her diet. For the last week she has been battling this. Its really strange that as the day goes on her stools are firm.
I have had her stool checked for parasites. It was negative and the vet wasn't too concerned since it was one stool a day. I have tried a bland diet and yogurt but nothing seems to be helping. I am beginning to think she might have a bacteria infection or something.
Has anyone ever delt with this before?? Thanks for listening.