Well, it can be normal this time of year for them to greatly slow down or stop eating as they start to brumate. BUT, in your case, I think there's a good chance there is more to it. We had the exact same pattern with our bearded years ago when we got it. About the same age, and the same TERRIBLE smell to their stool. I'd say there's a VERY good chance its coccidia. They just about all get it, as they came to you with it already in their system. It's like puppies having worms since they get them from their mother and have to be wormed.
Unfortunately, coccidia can be tough to get rid of. You'll need to not only have your bearded tested to confirm this is the problem, and to get a prescription, but you'll also have to clean everything they come in contact with and their viv thoroughly. There are only two ways to clean things to eliminate coccidia. First is heat, boiling or steam. Boil everything that can be boiled in water for a good while, like 30 minutes or more. Then, everything else needs to be cleaned exhaustively in ammonia. Bleach is useless against coccidia. You have to use ammonia.
As far as treating your dragon, there are two classes of drugs. Your vet may try to give you something like Albon or other sulfa based drug. Refuse it. Not only is it very hard on your dragon, but it is not completely effective against coccidia. The drug you want is Ponazuril. You may have a hard time finding it. If your vet doesn't carry it, get a prescription and contact Diamondback Drugs in Phoenix AZ who can compound it and send it to you.
The good news is that at least with ours, correctly following the above, we've never had another problem with coccidia after that first year. Best of luck!
