I don't see wet dirt feathers or a whitish discharge from the vent that would indicate Vent Gleet (see the link below for some photos).
When was the last time she laid an egg - are her eggs "normal" or does she seem to have trouble with soft shell/shell-less eggs?
Does the abdomen under the vent feel bloated/swollen or fluid filled? She may have an internal laying/reproductive disorder like Egg Yolk Peritonitis, Ascites, cancer or tumors. Any of the can cause a bloated/distended abdomen, going off feed, lethargy, weight loss in the breast, loose droppings that may look like it has egg in it, difficult walking/lameness and just a general unwell appearance. Having a fecal float to rule out worms, coccidiosis and bacterial infection would be a good idea as well.
Do the best you can to give her supportive care - see that she is drinking well, offer poultry vitamins to give her a boost along with some extra protein daily (egg, tuna, etc.).
https://www.tillysnest.com/2012/12/vent-gleet-prevention-and-treatment-html/