Bumblefoot is an infection of the foot, usually staph. It starts with a foot injury, a bruise or a splinter or a laceration, and it becomes infected, and it's usually identified by the blackened scab over the infected pus pocket.
Though wounds occur on the other parts of the chicken's body, we don't have a cute word to describe it. Maybe BYC should run a contest to label wounds or lesions found on the thigh, breast, neck, and back. I see a huge need here, do you? (BYC word junkies, alert!)
I think we all would benefit from a photo of this lump. It's not easy to see with feathers covering it, and it's not going to be easy to read a two dimensional photo to fully absorb a three dimensional lump, but here's what I suggest.
Take a spray bottle filled with water. Wet the area thoroughly so the feathers become saturated and they become clumped and plastered to the skin. Try to smooth the wet feathers away from the lump so the skin is visible. Use a light at an angle to the lump so the lump is shaded on the side away from the light. This will give the lesion contour and make it easier to see in the photo.
It sounds like a cyst which is often harmless. But if it's affecting her walking and causing pain, we need to try to figure out what it is and what to do about it.