Something's off

Little Man is sick, and symptoms are all over the place, so it's hard to figure out what exactly is going on. Since Friday evening, we've coping with the following issues: a yo-yo low grade fever (up and down and up and down), horror flick-worthy vomiting, loss of appetite and energy, fatigue and restlessness- so he's tired but never able to get some decent sleep. Luckily, he's been great about getting fluids in- though his tank is running low. Only one wet diaper today so far, and his lips are looking chapped. The craziest thing is that all of the symptoms come and go in waves- so we'll hit tiny pockets of normalcy, and I assume that he's finally broken through. An hour ago he was fry heaving and looking like he might pass out; now he's upstairs running around. The nurse line seems to think it is a short-lived tummy bug. If that were the case, wouldn't it have passed by now- seeing that he came down with this on Friday? What is going on???