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[Lord of the Rings] Why is Shelob hurt/blinded by the Phial of Galadriel?

So after learning more about LotR lore and how Shelob's mother, Ungoliant, fed off the light of the Two Trees, why does the Phial of Galadriel affect her if it contains the same light Ungoliant fed on. 

Is it because she is not from the Void and just a daughter of Ungoliant or is it something else?
gpt
Ungoliant eating the trees did two things - empower her, and remove their light from the world. When she ate them, her hunger overcame the pain she endured bathed in their light.

Shelob had been living in darkness for some time at the time of her confrontation with Sam, she was probably just taken by surprise and not willing to stand up to the light injured as she already was.