Roxy and Guinness' breed is prone to sensitive tummies. There have been a handful of instances in Roxy's 8 month life so far, where we've had to deal with diarreah and/or vomitting.
This has either been the result of a dietary issue or a virus. When they are vomittin or have diarreah, this is their body trying to rid itself of something that doesn't belong.
I learned through research and experience....
I don't know why, but that's what I read every where! She will be dehydrated so make sure that the ice chips stay in her water bowl. Replace as needed when ice melts. You should fast her for 24 hours (12 for a puppy- this can be done over-night). While she is fasting, you can give her pepto bismol or pepcid AC. This coats your dogs tummy and makes it less irritated. After the fast, give her something very bland, like boiled chicken and rice. Feed this to her as you would with dog-food. During her fast you can also give her gatorade or plain pedialyte this will help with dehydration.
Plain yogurt with live, active cultures can help restore the balance of good bacteria in your dog's tummy. I like this Greek yogurt because it has a few different types of cultures in it. I mix in a table spoon of this in Roxy's food regularly to maintain a healthy puppy belly. I double up when she is feeling sick.
She will need attention to her condition and lots of rest!