It could be anything.  And, I learned last night not to think that just cause you've wormed an animal that they don't have worms, LOL.  Apparently, these parasites are becoming pretty resistant to the common medications and doses. 

But, these symptoms could be anything so hopefully you've seen the vet.

I'm curious to know what was wrong if anything.  Hope he's OK.  I've sat for the breed and they are such nice little dogs-very easy to take care of.