There is a chance this behavior is egg related. She may be struggling to lay an egg. It's been my experience that a shell-less egg can cause a hen to feel "off", probably because the egg doesn't move at the rate of a regular egg, so the hen may be feeling "backed up". Try giving her one tablet of calcium citrate as it works quickest by stimulating contractions.
Hens go broody when you don’t want them to… and won’t go broody when you do.