Parrot has loss of appetite, increase lethargy but no fever.?

I gave my parrot only 1 chocolate cookie 2 weeks ago. He had diarrhea for 1 day. But the following day he seems to recover. His droppings has solid substance in it. Every thing looks normal again. But his behavior seems change. He is less talkative and less active. but he still has good appetite. still play and talk. But as days go on we noted he is less active, less talkative and likes to sleep but still have good appetite. So we thought it is depression. His appetite gradually decreases in the following days and so does the lethargy .droppings looks normal. Instead of squaking and preening when i got near him, he emitted like a whining sound. We went to a vet examined the stool and there is no parasite nor there was any fever noted. have anyone encounted this kind of symtoms. Is this related to the chocolate or not? or is it infection.