Every year there are stories about public pools closed due to fecal contamination or some kind of bacteria that has gotten into the pool. You didn't say where your daughter was at a pool. If it was at a pool that babies with swim diapers are allowed into, perhaps she swallowed some contaminants that have brought on a stomach issue. If she is still throwing up as regularly as she started out, by mid-afternoon, I would at LEAST call her ped. Or consider popping into an urgent care. With a regular stomach bug, once they have completely emptied the stomach of it's contents, the vomiting tends to slow down considerably. Going from say, every 30 minutes down to every 2 hours or longer. Then only when she has ingested something... and usually turns to diarrhea for a little bit. But if it's e.coli or something, I think the pattern is different.. I would research it further online or ask the doctor. The biggest concern is to watch for dehydration (sunken eyes, difficult to rouse, etc). Small sips of something without a lot of sugar every 10-15 minutes until she can keep things down better. (diluted gatorade works well for this). Then maybe offer her a few goldfish crackers or saltines and see how that goes.
I doubt the juice at the party caused anything.. Food poisoning sounds much more likely to me. If there was some food that sat out too long and maybe went bad? Food poisoning makes me feel like that (like a rock in the stomach for awhile before the vomiting starts). And usually there aren't really other symptoms besides stomach ache and nausea. Don't force foods... let her tell you when she is ready. Offer, but don't insist. A nice Disney flick with a blanket sounds like a nice way to spend the afternoon.