First, don't worry, this behavior is entirely normal thing that many cats do. I once visited a friend of mine and she warned me that her cat would visit me on the bathtub ledge between the two shower curtains during my shower. He did, but didn't talk to me, so I was super startled to see him there when I pulled back the curtain!
There are actually a couple of reasons she could be exhibiting this behavior, or it could be a combination of multiple reasons.
If your cat normally follows you around the house, this is just a normal part of her keeping tabs on you. As Dan S says, she wants to join you but doesn't want to get wet, so she's telling you to get out of there.
Another possibility is that if you coo at her and give her attention (verbal, eye contact, etc), she's looking for that attention and knows if she yells at you long enough, you'll give it to her.
Cats are also creatures of habit. If you get up in the morning and immediately jump in the shower and get dressed before heading downstairs to feed her (or whatever), then she's trying to rush you through your morning routine and get to the part where you feed her faster.
If this behavior is bothering you, the best thing to do is close the door so she can't come in, or ignore her when she does talk to you in the shower. We broke one cat of the habit by switching to an opaque shower curtain so she couldn't see us anymore (and ignoring her cries), so that's another option.