Like bitsy said, corn snakes kind of flatten and widen their necks when they feel threatened. It could be described as "puffing out" a little bit,too. I've seen my corn snake doing it when he's startled. Sometimes he does it when I'm letting him roam the carpet. I think he's making himself look bigger in case he encounters something frightening in such a big, open space. One time, we were cleaning her tank, and put him in a separate tub to do it. It agitated him, and when my boyfriend went to pick him up, he bit him. He had "puffed" out his neck before and after. Sometimes he only puffs out one side, and it looks like when a human has a swollen lymph node in their neck!
I don't know anything about them getting puffy before shedding, but I'm a pretty new owner, so that would explain why I've never noticed it. I'll have to inspect mine next time he's about to shed. It makes sense that it would happen, though. The first time mine shed, he was making weird puffing noises when he breathed. Posters on here shared with me that, since they start their shed at their heads, sometimes flaps can form near the nostrils, making that sound.