The thing is, it's not amnesia. She has memories - of her mundane life in Storybrooke. You'd react the same as her if someone came up to you in the street and started calling you Belle.
Regina had Belle locked up in the psych ward of the hospital, and she was only freed by the Hatter after the curse was lifted. So her only memories were probably being locked up and getting injections to keep her docile.
Exactly. All that stuff about meeting Ruby and becoming the town librarian happened after
she regained her memories of being Belle. We have no idea what that forgotten inmate's "Storybrooke" name or history were.