We've been really lucky with DD4 in that she's rarely got ill but she's finally been hit by some sort of vomiting bug (no diarrhoea). I've got no experience of this but currently she's been vomiting every 10-20 mins for the last four hours and best as I can tell can't keep water down. At what point do you worry? I know it's not yet but her vomiting hasn't got any less frequent at all.