I'm so sorry to be reading this and please accept my deepest condolences.
Growler has some good theories here and being so tiny likely didn't help at all.
When my cat Ziglet passed away a couple of years ago, it also happened very very quickly (within 2-3 weeks).
I just wanted to thank you for adopting a senior cat, you made Lucy's short time with you special and that is a very beautiful thing.
Sweet Lucy
Sincerely,
Marko