The TARDIS can't tell the shape of a dream before she is in the middle of it; if she could, she'd have definitely shied away from this one like a spooked horse. So when she slips into the dreamscape, she isn't prepared for the onslaught of terrible memories, for how accurate and overwhelming the warped dimensions feel, for remembering how terrified she was without the Doctor in this horrible timeless universe, how desperate to get her message through to him, how scared of Rassilon.

The memory is so intense that she doesn't even manifest as a human, but in the shape and place she had been here - the blue box at the center of the maze. It's only when she feels someone bump into her that she's startled out of her moment of shock and quickly changes her appearance to look around and see who else had the misfortune of stepping into this dream.