Hello and welcome to pet lovers! I am sorry your dog is having this problem, it's clear you take very good care of him.
Dasequin has no side effects, it is a harmless supplement. I would have to conclude that it's simply coincidence that this problem started around the same time you started the joint supplement.
What does your vet say about it?
<edit> The link you provided is a question about rimadyl, which is not the same thing as dasequin at all. Rimadyl is a NSAID, which is a pain medication. Is your dog taking rimadyl too? Rimadyl is a drug, and as such can have side effects for some dogs.