My concern is in the first post, you said that it was hot in the house - heat stroke can certainly make a chinchilla lethargic, minimally responsive, not want to eat, drink, and produce little/no feces. I don't know what the temperatures were like and what the chin is used to.
A pound is 454g. ;P
A lot of chinchillas are in the 500-700g range. Just like not everyone grows to be 6 feet tall, not every chinchilla grows to be 900+g. Last time I weighed my chins, one was about 600-something and the other about 700-something g (it's been a while, and I forget... I should probably dig out the scale again). People are so obsessed with weight, but that's not the only thing to consider.
And one of my chins certainly does get fat if unlimited pellets are provided. He gets bald rolls under his armpits, and I can't feel his ribs at all. Thus, two chinchillas get to share about 2-3Tbsp of pellets a day, and I free-feed the hay, and that seems to keep it under control. ANY animal can get fat. Chins might be less prone to it than, say, dogs or rabbits, but they can do it.
Good luck with your chin.