I don't want to scare you but I would call the vet, here is some information:
Grape and Raisin Toxicity in Dogs | VCA Animal Hospitals
Because of her/his size if it were multiple grapes, it could enough to trigger something.
If you don't have access to a vet or emergency vet, I personally would induce vomiting (and if you know it's been just recently).
We had an incident with Pluto and macadamia nuts. He seemed fine, but in the middle of the night he started crying, and then couldn't walk. His hind legs seized up. It was the toxicity moving into his system. We rushed him to the emergency vet right away in the middle of the night and they treated him with coal stuff. He ended up being fine within a day but it was a close close call and if we hadn't been alert that night, I don't want to think about what could have been.
Not wanting to scare you but take it seriously. The vet said sometimes animals won't react, but you never know when they will.
I'll be praying that everything I okay and there's no reaction.