Sudden death of my cockatiel

I've had a wonderful female cockatiel since 1996. About 2 years ago, she started getting crusty stuff around one of her eyes, and we took her to an avian vet. He tested her for Clamydia, which she didn't have, removed the crusty stuff, gave her a shot of antibiotics, and gave me eye drops to put in her eye every day for a couple of weeks. That seemed to clear it up.

The same eye problem reoccurred recently, so we took her to the vet this past weekend thinking it would be the same thing. She was acting perfectly fine outside of the crusty stuff around one of her eyes. When we got to the same vet, he held her and looked at her eye (she doesn't like to be held where she can't move, so she wasn't happy). When he tried to remove the crusty stuff (which came off pretty quickly like a small scab), she all of sudden went completely limp. It was like she passed out; only she never woke up. She was dead just that fast.

I'm heart broken and can't stop wondering what could have caused that. Could she have gone into shock or had a heart attack from the stress of being examined? Any information you can share would be so appreciated.