That link is to a forum here where you can post chicken illnesses.
My barely one-year-old hen acted like yours, so I brought her in. She was feverish on her back underside, but nothing abnormal at her vent area, so she's got an infection in there, but not necessarily internal laying. I gave her turmeric and black pepper in her mash for inflammation and ordered 25 capsules of 250 mg amoxicillin from
https://www.twincitypoultrysupplies.com/ I just gave her her second dose with a syringe and the fever is gone, but she's still tipsy so a ways to go yet.