An accident with the Mirror Portal ends up with Twilight stuck as a small pony in the Equestra Girls' world. Her friends can't get over how cute she is.
Sometimes the girls at CHS forget Sunset is a pony, so when Rarity finds a rather odd bottle of shampoo in Sunset's bathroom, the confusion is understandable.
Why is Celestia laying on the floor in Twilight's library? Why does she seem so confused? Why does she look different? WHY IS THE PORTAL MIRROR STILL ON?