It could be a number of things.
She has not been wormed and I'm thinking I'll try that today....
There are no signs of mites but we do have a ridiculous amount of mosquitoes here in South Florida...could that be a factor that would only affect one bird?
Her crop feels empty if slightly squishy much like the others but without the lump of food the others have.
She absolutely could have eaten something rotten as were in the middle of mango season and there's rotting fruit everywhere.
She ate a little food this morning and I saw her drink as well. I started the whole flock on Corid yesterday to rule that out and I'll worm them also. Any suggestions on a good wormer?
Thanks so very much for helping me solve this mystery!
Is she overheated?
When was the last time she laid an egg?
Any chance she ate anything rotten/moldy?
How does her crop feel, hard, squishy, doughy, etc.?
How does her abdomen feel - hard, soft, fluid filled, etc.?
Any signs of lice/mites?
Has she been wormed?
If you think she is overheated, move her to a cooler area and offer cold water - the electrolytes are ok for a couple of days. If she feels really hot, then you can dip her legs, feet and lower body in
cool water. Heat stroke may take a couple of days to recover from. Once she is hydrated, offer wet feed and some egg, tuna, mackerel or meat.
http://www.the-chicken-chick.com/2012/04/beat-heat-helping-chickens-survive-high.html
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx