Sudden Death!

So my flock has been doing great - no signs of any problems except for one girl that was laying soft shells, really since she began laying last fall, with an occasional hard shell but more often than not soft. I would single her out for vitamins and oyster shells but no change in her eggs. Today I discovered her dead in the yard! i examined her and no sign of obvious trauma even though she was kind of flat on the ground. Her eyes were closed but clear, no missing feathers. I do feel a tiny peas size knot in her neck, like a little pepple - like an and am wondering whether she could have choked on something? Eyes clear, comb pale but she's always tended to have a slightly paler comb than the others. Not egg-bound, normal poop residue on feathers, no lice or mites - nothing visible. No tumors, lesions, scabs - just soft shell eggs when she does lay. The only thing I can guess is she ate something poisonous or something got lodged in the esophagus. After palpating and examining another of my other chickens I don't feel this pebble.
My question is can a chicken choke on something? Any ideas? Could the soft shells be indicative?

send your girl to a lab somewhere around you, you say there is a knot in her neck I'm thinking it is a tumor, which could be mareks. not trying to scare you but if she was fine and then suddenly dropped there is something up.

They were all vaccinated for Mareks as chicks and no exposure to any other fowl. Doesn't that rule it out? How is it transmitted? All the flock came together as chicks and all were vaccinated.

It's a small knot - feels like a tiny rock pea size

unless you vaccinated them yourself, occasionally you can get one that has been accidentally skipped, it is transmitted by wild birds, and sometimes they can get sick even if they are vaccinated.