Just wanted to chime in that mine do the same thing. One of my hens has chronic issues with soft-shelled eggs, and she tends to lay them on the floor, where the hens will all eat them. Another of my girls lays eggs with good hard shells on the floor, and no one ever bothers them. They walk on...
Can you feel an actual lump on her back? If you have calcium citrate pills, the human ones, I would give her one, just in case she is egg bound. It won't hurt her if she's not. Has she been acting normal the past few days?
What I'm wondering is if your hen has an actual lump or if that's just her feathers poofing up. This is my hen kind of looking the same, and she was perfectly fine.
Your hen clearly isn't feeling good, which accounts for her just standing there with her tail down. How old is she, and what...