My kitten got sick.?

I am 16 and I just got a new kitten for my birthday like a month ago. She was fine one day then the next she was acting like she didn't feel good. We found out that she had fleas from our dog that comes in and out. We called the vet and they said just to give her pedialite and offer her plenty of food and water wich we did . I think she was dehydated. We did everything we could.But it came on so sudenly she died shortly after was this painfull for her?? I am very upset and hope that it was not.