My 4. month old vomited yesterday and this morning has she got the stomach bug and what should i do?

Monitor/fluids: This is relatively common & can be minor or get worse. As long as the kid takes in enough fluid to wet every 6-8 hours they are hydrated. If not wetting in 12 hrs they are dry & need to be seen. Pedialyte in small (1-2 oz) amounts every 10 min or so can be advanced as tolerated & replace losses. Breast milk is fine as a feed. It should pass in a few days & is often followed by loose stools.