'I'm fine.'

Alternatively titled "Waifu Mode"
alternatively "How Many Horrific Car Accidents Wa-"
alternatively "BAND-AIDS"
alternatively "I Should be Working on the Comic"
alternatively "Not Going to Make a Plot Joke"

By the way, this is still just a joke, but-

As a canonically acceptable explanation I like to believe that Rainbow was assigned to one of the harder weather tasks, as a leviathan storm destroys Ponyville... or something.

She's telling Scootaloo that "she's fine", obviously because she realizes Scootaloo hails her as a hero.

And heroes don't fall.