I agree that her abdomen looks swollen and that is the reason her butt feathers are getting soiled. Their body should be shaped so that there is an overhang below the vent so that poop drops clear. If there is swelling below the vent, there is no overhang and the poop snags the feathers below and becomes soiled.
Your bird has bright comb and wattles which suggest that she is ovulating and she is visiting the nest box but you not getting eggs from such a young bird in several months is telling. That information combined with the abdominal swelling strongly suggest that she is suffering from a reproductive malfunction, most likely either laying internally and/or salpingitis (an infection of the oviduct where egg material and pus form what is referred to as lash egg). Unfortunately both ailments are likely to prove fatal in the longer term, if she doesn't succumb to fly strike before you get back. I hate to say that when you have probably already left and can do nothing about it. Perhaps you can get your chicken sitter to apply a good layer of greasy fly repellent each evening whilst she is roosting. Some horse wound products can be good. I have a pink salve for my horses called Protection Plus that is a greasy gunk that contains citronella. It can be plastered on and works great for scaly leg mites on chickens but would be ideal for this situation. I'm in the UK so not sure what similar products you would have across the pond.
I hope I am wrong but that combination of symptoms points heavily in that direction.
Your bird has bright comb and wattles which suggest that she is ovulating and she is visiting the nest box but you not getting eggs from such a young bird in several months is telling. That information combined with the abdominal swelling strongly suggest that she is suffering from a reproductive malfunction, most likely either laying internally and/or salpingitis (an infection of the oviduct where egg material and pus form what is referred to as lash egg). Unfortunately both ailments are likely to prove fatal in the longer term, if she doesn't succumb to fly strike before you get back. I hate to say that when you have probably already left and can do nothing about it. Perhaps you can get your chicken sitter to apply a good layer of greasy fly repellent each evening whilst she is roosting. Some horse wound products can be good. I have a pink salve for my horses called Protection Plus that is a greasy gunk that contains citronella. It can be plastered on and works great for scaly leg mites on chickens but would be ideal for this situation. I'm in the UK so not sure what similar products you would have across the pond.
I hope I am wrong but that combination of symptoms points heavily in that direction.
