Amnesia

Rainbow Dash, still in shock, takes a few steps back. She then leaves for a few minutes and returns with the doctor.

"...and then she said she couldn't remember me!" Rainbow Dash said to the doctor.
"Okay, alright." the doctor said, and then he turned to Fluttershy. "So Fluttershy. You don't remember her?"
"No..." Fluttershy said to the doctor.
"Well, do remember everyone else?"
"I think so. That's Twilight. That's Applejack. That's Pinkie Pie and that's Rarity." she said, pointing to everyone in the room.
"Hmm... Interesting. It seems that she can remember everyone except you, Rainbow Dash." the doctor said.

Rainbow Dash leaves the hospital and starts flying back home. In tears.