She doesn't know what the dust is, it's not like she researched being a fairy before she got turned into one! One thing is for sure though, and it's that Molotov's fairy dust does not make people fly. It probably gives them a rash or something, she really has no clue.

Molotov had flown to the gym, which is a little more disorienting than walking there, and she's distracted by her own thoughts (pouting) about her current situation. So it's only once they're actually on the second floor that she notices, seeing the room numbers, and she starts yanking at Spike's hair, trying to tell him that he's an idiot.