This fish of yours might be having some internal parasites or worms, which is even more likely since you got him from a pond
Go ask at the pet store for something to treat these worms with, I don't think it's too late! (Just in case, I'll check out the symptoms and come back with more treatment info if I find anything that might be of help!)
Oh, and while you're at it, go buy yourself some frozen peas and some freeze-dried live foods (: Goldfish thrive when they have a varied diet!
And it's easier to avoid constipation if you feed the fish with thawed peas (: (Pop them out of their shells before feeding)