Something similar happened to my baby boy
Gizmo, I had fed him Eukanuba lamb & rice ever since he's 1, and I decided to change his food to Eukanuba Yorkshire breed specific, what a mistake I made, Gizmo one night out of the sudden, 3 wks after I made the change started feeling ill, his vomit had tiny specs of blood, & his stool as well, I was very concerned and rushed to the vet the next morning, the vet, just analyzing by eye told me his tummy was too hard, and that this was an intestinal case... He then ran blood work, took x-rays and tested for worms... He had an intestinal infection, due to the change of food, I felt like such an Idiot!... He put him on i/d and has recovered... If I would have waited, and continue to ignore all the signs, perhaps I would have lost him!! All pups are different, My baby
Gypsy loves the food, she tolerated it very well... Dont wait and get a second opinion... Sometimes their stomachs are very sensitive, and the slightest change can throw them off.