Canterlot castle, four years ago

Twilight happily trotted her way through the hallways of Canterlot Castle, returning to her room after a lesson with Celestia. Today’s lesson had been on teleportation. Despite the fact Twilight almost trapped herself in a wall, she was still in high spirits. At the moment, her room was located in the royal wing, just down the hall from where Princess Celestia's room was. Twilight was looking forward to the restoration of the old observatory, as the princess had promised to make it her own place, as an early birthday gift.

The Royal wing was fine by her in the meantime, as it was usually quiet down there, mostly due to all of the security. It helped her with her studying. Usually the only ponies she would have contact with were the royal guards, who she knew quite well, the princess and Spike. On the weekends she would leave the castle and spend time with her family or Princess Cadance, and even more occasionally she would meet up with her school friends, Lemon Hearts, Minuette, Twinkle Shine, Lyra Heartstrings and Moondancer.

Twilight trotted down the hall; passing a couple of guard patrols, who she briefly greeted before moving on. Twilight soon came to a door at the end of the hall, behind which was her and Spike's room. Opening the door with her magic, Twilight slipped inside and closed it behind her. Her room was fairly large, with an abundance of bookshelves and a big purple bed in the corner. Next to her bed was a basket with a few comics scattered around it.

Spike wasn't in; he was likely out causing trouble for the castle staff as usual. Twilight carefully placed her saddlebags onto the bed and took out the books inside. She began to arrange them on her shelves in a neat fashion, arranged in the order she intended to read them.
But the book she was levitating, a rather large tome, suddenly slipped from her grasp, and made a loud thud as it impacted the floor. Twilight held her head as she began to feel a little faint, she didn't notice green flames whip around her, tearing away the visage of the lavender unicorn she knew, but she did see a green flash.

"Whoa, what was that?" Twilight muttered to herself as she returned to her senses.

She looked around, trying to find the source of the green flash. Twilight's eyes fell onto her ceiling height mirror, upon which a black pony-like creature was staring at her.

"Eeep!" Twilight shrieked out, falling over backwards trying to scramble away. She righted herself, and dived behind her bed. After a few seconds of nothing, Twilight stopped panicking. Deciding to be brave, she peeked up over her quilt. As she did so, the mirror showed the same black creature appear over the top of the reflected bed. It was at this point she realised the creature she saw was her.

At first Twilight sighed in relief, only then hit with a wave of realisation, and began to panic again.

"Why am I an insect pony … thing! I have holes in my legs! My horn is crooked!"

Twilight began rambling to herself; trying to make sense of what was going on, and she was failing miserably. Her thoughts changed from what was she to what would happen if somepony else found out. Would they experiment on her? Banish her?

Not likely, but the scenario crossed her mind anyway. Twilight remained in this state for a solid fifteen minutes before another voice brought her back to reality.

"Twilight? Are you OK?"

Twilight held her breath and closed her eyes, as she heard the door open and the figure of Princess Celestia enter.

'Here it comes – She’s going to banish me, she’s going to…'

"Twilight, what's wrong? You look terrified!"

Confused at the princess' reaction, Twilight opened her eyes, the mirror revealing she was once again a lavender unicorn.

"Oh, nothing, it was nothing; just another one of my panic attacks," Twilight said sheepishly.

Celestia sighed and nuzzled her student affectionately. "You know that if you need to talk that I’m always willing to listen, and you know where to find me."

"Thank you, Princess."

Celestia smiled before giving Twilight another quick nuzzle, turning and leaving the room. She had barely closed the door behind her, before Twilight ran up to the mirror.

"What the hay is going on?" Twilight muttered, thinking with more clarity.

She looked at her own reflection; and focused on the memory of how she had looked a just a few minutes prior.

As she did so, green flames once again whipped around her, and the black creature returned.

Then she focused on her unicorn self, and once again, she became just that.

"Huh, I'm a shape shifter? I wonder if I could... no Twilight, I need to focus please."

Twilight returned to her 'creature form' and observed herself intently.

Seeing herself like this... it felt natural.

"This can't be normal, more research is required!"

Twilight shifted back into her unicorn form, picked up her saddlebag and quickly rushed out of the room towards the library, both curious as to what she might find... and terrified.

Author's Notes:

I may have these flashbacks between chapters every now and then, though I'm not 100% certain.

This was before she got her own place that we saw in Season 1 and 5.