going thru some things with one of my dogs, believe me, it could be anything.
Are you in an area where there are ticks? There are sooooo many tick borne diseases now, mostly undetectable, you get alot of false negatives, that its sometimes best to just throw them on an antibiotic.
Personally, I"ve never heard that flagyl can cause stiffness. Not to say it isn't true, I've just never heard it.
Glad he's feeling better,
diane