You have been given some great advice and it sounds like she is on the mend.

We had a gastro bug a mnth or so ago and it started just like you described the really awful nappies and then the vomiting, he the vomiting for about 24 hrs (the first 5 were the worst) and runny poos for about 2/3 days. When he started back on 'normal' food he didn't eat as much and I just let him have free access to boob (we were down to 2 feeds through the day).

He also slept alot more then normal for example he started to have 2 sleeps again and they were long sleeps where as normal is 1 afternoon nap. He definitely wasn't as active but I wouldn't class it as lethargy as such. If it had gone on for much longer I probably would have taken him to the dr just to check his hydration levels.