Why does my cat freak out when I'm in the shower?

My cat, Sunny, is a complete freak. When I get in the bathtub she blows a fuse and meows in a panic. She chews the edge of the plastic shower curtain and bats at it, crying the whole time with her eyes opened wide. What gives? I know plastic can make cats act weird but this weird? She was a shelter cat almost her whole life before me. My other kitty doesn’t do this at all.
Any ideas?