Silent Ponyville 1:
After the Grand Galloping Gala, Pinkie began to have nightmares. Terrible, awful nightmares that made her terrified to fall asleep. Being the party pony she was, she tried to brush them off and ignore them. To be happy with her friends.
When they became too much though, she went to the only pony that could help her. And Twilight did the only thing she knew how to do.
She cast a spell that let Pinkie confront why she was having those nightmares.
And so Pinkie entered an empty, fog filled version of Ponyville. Alone and confused, she wandered the town until she realized she wasn’t alone. There were monsters lurking in the fog, wanting to kill her.
Confronting the monsters, she learned of her past and why she was having those nightmares. Reuniting with her sister Octavia at the Grand Galloping had triggered memories of her past to come flooding back and haunt her.
The simple answer was that she was afraid. Afraid that she would become a monster once more, and lead her friends or family into the maws of death. That she would be the one to get them killed.
Just like how she had gotten her youngest sister, Bellamena, killed.
She had forgotten about Bellamena, in order to save her own sanity, to try and live a normal life, but she couldn’t live that way anymore. With the help of Twilight and her Sister Octavia, she was able to come to terms with Bellamena’s passing. She was not the one who killed her, that madman, The Colt, had been the one to do it.
And so, Pinkie was able to put an end to her nightmares and life was able to return to normal.
Or at least, that was what she had hoped.