Is my dog sick or just stressed out?

Honestly many dogs experience separation anxiety when being boarded, new place, new folks, new dogs. Where are my humans and my home? What troubles me more is the kennel didn't notify you immediately when she got ill. I'd bet she is dehydrated and may need an IV infusion from your vet. Hope your pup recovers quickly

In addition to the separation anxiety reaction that many dogs have, if the boarding place changed the dog's diet at all (even if it's same or better quality food and treats), that very frequently leads to upset GI with throwing up and/or diarrhea. If they failed to inform you of that condition for more than 3 days, I wouldn't recommend Ever going there again. Although the best care providers are prepared for those types of occurrences and often remedy prior to pet parents' return, it is considered a best practice for them to inform you. After three days, the dog runs the risk of dehydration and needing professional (vet) care.

In the future, your dog may do better with a home sitter, who you have stay with the dog at your home. Or possibly, a different boarding situation. Either way, I'd suggest you allow your dog (primarily) and yourself get to know the care giver by booking shorter duration care services, whether that's walks or day care, first to build a relationship. It might not make much difference to you, but it often does for the dog.