TAKE YOUR BIRD TO THE VET RIGHT AWAY!
My black capped conure had similar symptoms a few months ago and it turned out he had swallowed a tiny .2mm (yes, POINT two millimeter) piece of metal that was poisoning him. He lost 10% of his weight in a couple of days and I nearly lost him. It required x-rays to identify the problem, and two oral meds and two rounds of injections to bring him back to health.
This isn't something to mess around with. Birds are prey animals and so they hide when they are sick. One of the best ways we bird owners can tell if our birds are ill is by keeping an eye on their stool. Have your vet take crop and stool samples to check for infection. You'll be much happier spending the money on a vet visit and finding out it was nothing to be concerned about than you would not spending the money and losing your bird.






