Could have brought hand up too fast. Could be someone else is doing something to her and has hit her in nose.
Could be she didn't see hand and was startled.
Could be carrot didn't smell right.
Could just not feel like being messed with.
They aren't robots, and may well wake up in mood they don't want to be bothered, or don't feel like doing something they did days before.
If this worries you, don't ever get caged birds...what one will eat one day, the next day gets tossed out of the cage.
Horses make me a better person.