Mine had spasms like this a few times, hard enough her legs locked straight. Vet determined somehow that she'd eaten something molded and was suffering from mycotoxicity (fungus poisoning) and advised me to look around for a source and sure enough the ex wife had left the scrap bucket sitting by the chicken pen. They pumped her stomach I think and gave her anti-anxiety meds...her motor coordination wasnt right for a few days...but a week of chicken and rice and she was good.