It's possible it picked up something at the day care. So start by taking it and fresh urine/fecal specimen to a vet.
No dog is un-trainable some may require more patience, consistency, diligence and time (minimum 6-18 months, if done properly, to start) on your part.
Obviously she wasn't yet trained when you went on vacation.
Did you spend the above minimum time to train it?
As mentioned some dogs may take more than the minimum time and if not done properly some will never be trained, not the dogs fault, the owners.
Did you clean all the soiled areas immediately and properly by first removing all the liquids/solids and:
http://i40.photobucket.com/albums/e224/GLLNTKNIGHT/carpeturine_zps07ea7542.jpg ?
First and foremost, you're suppose to be constantly supervising her, contain when you can't, immediate correction when caught in the act. She can't eat it if you supervise/correct and immediately pick it up.
She may have been confined in a crate at the day care, not exercised properly and soiled her crate and they failed to clean it up immediately, hence she started eating it.
You must also realize once she is trained, it doesn't stop there, it's your responsibility to continue enforcing what was taught on the daily basis for the rest of the dogs life, 12-17 years
Good luck..