That's so sad... Twitter also passed away very quickly, one day he was fine and the morning after I found him very apatic in his cage, he died a couple of hours later. The vet didn't know what it was, but I later found out that the pellets he was eating (Harrisson's), and that are so good for budgies, are too rich in protein for parrotlets, they can't process the thing. He probably died from kidney failure. He was also quite thin, which always amazed us, considering the amounts of food he was eating, the vet said he probably stopped drinking and then couldn't assimilate his food. I know he was drinking but then, what I found about the protein thing is that it gives that kind of symptoms, the kidneys are failing, thus they don't get enough liquids in their blood, thus don't assimilate the food.
I just hope he didn't suffer for months without me noticing and because of my ignorance...