It's a reference to one of Twilight's panic attacks... the idea that by not living up to her ideal Princess Celestia's expectations, Celestia would punish her by denouncing her and sending her back to "magic kindergarten".... Trixie, of course, would already have been there in Twilight's mind... since she would have fallen from her imagined Celestia's vaunted status of protege. Trixie, likewise, being the perpetually angsty/semi-antagonistic character that she is, would be imagined to have noticed Twilight and, though now knowing Twilight can whoop her, acknowledging her in a dry, albeit confrontational manner.



TLDR: Twilight freaks out, like in the episode, and imagines finding herself at Trixie's level of 'normal' rather than super-smart-princess-protege.