What Causes Stomach Infection In An Infant?

Why is my one and a half year old having rotten-egg burps, a bloated hard tummy and creamy yellow bowel movements with lots of farts? She vomited a little twice 3 days ago and has been not quite herself since 4 days ago but mostly in good form. After one day off food, is back eating most things. Have also noticed her falling over a bit more than usual. Someone suggested this could be an ear infection? Could it be teething? Or a viral tummy bug? The burps started just today.