Hi Jen, sorry to hear about your mucky hen, poor girl. How old is she, when was she last wormed with Flubenvet, what colour are the droppings, and is her crop OK? ( ie not bunged up with hard stuff, or else not soft and squidgy.)
She may have some kind of gut infection, especially if her droppings are green, and if this is so, a course of antibiotics may be indicated. Or, if not wormed in the past 4-6 months with Flubenvet, she may have a worm burden which is affecting her digestion. If her crop is Impacted, or if she has sour crop, she wont be digesting her food properly. Hens can actually be feeling under the weather a bit, but try hard not to show it, so if anything is abnormal its good to take it seriously before it gets worse.
If you could give a few more details, a better picture would perhaps emerge.