UPDATE:
So I flipped her on her back and inspected the area closer. I didn't see signs of mites or lice.
However, here are my other observations:
1. Not sure if she was scared or what, but I felt/saw a milky-looking liquid drip onto my leg when I was inspecting her. Could this just be from her being frightened? Do chickens pee? Or could this be some sort of discharge?
2. The actual bald spot is 3-4 inches below the actual vent. I felt the skin on the bald spot and it is shiny, red and white blotchy, and is flaky/peeling in some spots. Unfortunately that area did seem a little swollen. I picked up my other Golden Comet to feel her in that area and compare. It was hard to tell, but Henrietta's did seem swollen.
3. The vent itself was mildly moist (seemed normal to me - a novice). I did see what looked to be dark, dried poop below it. But I assume that is somewhat normal?
4. I don't think she is molting. She is still laying eggs daily and this is the only spot that lost feathers.
With that said, what seems most likely to you guys? I am totally at a loss here. This is all new to me.
Please help if you can because she is my most beloved hen.