It looks like it could be an abscess, the fact you're saying it's hard makes it more likely.
You can take a cloth and warm saline and apply it until it cools, then dip the cloth into the warm saline and repeat until the rat stops letting you do it. OR you can fill a small tub/container with warm saline and do a sitz bath (let her sit in it for as 5+ minutes or as long as she will stay). Rinse her off afterward.
If you have epsom salts, they work better than table salt. You want the saline to be 1%, so 1 tsp. of salt per 2 cups of hot/very warm water.