You may want to get another stool test done. My son's boxer had coccidia, which caused weight loss, loose stools, etc. The first test nothing showed up, with the second test the coccidia was there. The vet explained coccidia is one that is difficult to diagnose because it's not always in the stool sample provided.
Definitely check out what Crystal and Ann have mentioned but it may be something simple to treat like the coccidia. Especially since he acted fine until after a couple of days at doggy daycare.
__________________  ~Ruby, Reno, Razz, & Jack~ |