Unfortunately swelling in the area you describe usually indicates salpingitis which is an infection of the oviduct which causes egg material to bind with pus and solidify into a lumpy mass called lash egg. Sometimes in the early stages of the ailment, the hen will "lay" a lash egg and it will be found in the nest box or sometimes in the run. Other times it just impacts the oviduct until eventually it gets so big that it constricts the gut and the bird's digestive system gets stopped back. The crop will usually become slow or sour and the bird will go off her food. She will usually die from toxic shock because she cannot eliminate faeces and only white liquid urates will be passed. Unfortunately at this stage it is usually too late to do anything but this ailment is often fatal even if caught early unless antibiotics are administered or major surgical intervention (hysterectomy) by a good avian vet, but the latter is very risky and expensive and the after care involves hormonal implants every 3-6 months to prevent further ovulation at $100-150 a time.
Another possibility is a tumour or internal laying although the latter usually causes the swelling to be lower hanging between the legs.
Sadly hens are particularly prone to these reproductive disorders, especially with high production birds like red sex links etc, once they reach 2+ years old as their reproductive system is well worn with constantly producing eggs without a break.
I'm sorry to be so bleak in my suggested diagnosis. Does any of the above sound like it ties in? Can you post a photo or her back end? How old is she and what breed?
If I am right, it may be time to consider euthanizing her if she is no longer eating. Does her crop feel full and squishy or empty? I would certainly recommend you at least get a second opinion before you follow that suggestion though. It is not something I suggest lightly but the location of the swelling and it being red and her going off her food is pretty telling. There is a video which shows a necropsy of a bird with this condition which is actually a propaganda video for veganism which I don't personally adhere to but unfortunately it does illustrate the hopelessness of this condition. I can post a link if it will help and you are not squeamish.
In the meantime, keep her somewhere that is a comfortable temperature where flies cannot lay eggs on her and you can monitor her and check to see if her crop empties and what her poop looks like and if she is eating or drinking at all. Some poultry supplement like Nutri Drench dripped carefully into her beak a drop at a time may give her a little boost but be very careful she does not aspirate any into her trachea.