This can be common in some flocks, and it could be that it just looks red and swollen because others have been picking out her feathers. Can you post a picture? Feather picking can get out of hand, leading to vent pecking and damage. I have a hen with a large mass under her vent for the past 2 years, and she probably has internal laying masses of egg material or possibly ascites (fluid in the lower belly.) If it is just from feather picking, I would try some BluKote to mask the bare skin every 2-3 days, or some bad tasting cream or ointment (Vicks, BenGay, Nustock, BagBalm) daily to help prevent pecking. Be sure to check her skin under the vent and elsewhere for lice or mites.