There comes a time when an alicorn. whether ascended like Cadance or Twilight or born, like Celestia, Luna, and Flurry Heart, goes through a change in their bodies, as they become fully attuned to their domain.

That time has come for Twilight to grow fully into her element. Unfortunately, no amount of advice from her fellow princesses can prepare her for what exactly the process will do to her, for it is unique to each of them.

On a totally unrelated note, Twilight doesn't want to be an alicorn anymore.