
Jade,
You should *always* keep water in your dogs bowl and available. If you only got your dog two weeks ago and the vet said she is in good health, she may just be settling in to her new environment. It can take some time before a dog is comfortable and their true personality comes forward.
If her poo is runny she may have been intolerant to the food you switched her on - or she may have had the same problem with Pedigree. I don't know where you are based, but if you have a Petco or some other big pet store near you might want to try one of the brands of food marketed as hypoallergenic (no wheat, soy, etc.)