Could just be stress from the change of environment, but I would take a fecal sample just in case. Coccidia definitely can lie dormant in the system and be triggered by stress. Many owners pick up their dogs from boarding and then discover within a short time they have coccidia. They blame it on the kennel, but it's a good possibility that it was already in their system prior to being boarded.
It could also be giardia with the anorexia. Any mucous in the poop?
If it is coccidia they can give medicine to stop the little critters from reproducing, but the dog's immune system must overcome the problem so look into giving an immune system enhancing vitamin and as with all antibiotics give Benebac (good bacteria to replace what the antibiotics kill off) - sold in tubes for about $6.00 and more concentrated cultures than found in yogurt without the risk of digestive upset from the lactose.
From
www.thepetcenter.com
We do not have any medicine that will kill coccidia; only the patient’s immune system can do that. But we can give medicines called “coccidiostats” which can inhibit coccidial reproduction. Once the numbers stop expanding, it is easier for the patient’s immune system to “catch up” and wipe the infection out. This also means, though, that the time it takes to clear the infection depends on how many coccidia organisms there are to start with and how strong the patient’s immune system is. A typical treatment course lasts about a week or two but it is important to realize that the medication should be given until the diarrhea resolves plus an extra couple of days. Medication should be given for at least five days total. Sometimes courses as long as a month are needed.