Pinkie Pie and Rainbow Dash weren't the only ones to wet their snow pants that day. And if you were as nervous as Fluttershy and have been "pinned down for hours with no reinforcements and no handwarmers" (and no access to a bathroom), you would too. At least it warmed her up a little, though her hands seem to need it most.

And don't worry, she'll be fine. Once the war's over all she needs to do is get home and change, and take a nice hot shower she'll thoroughly enjoy, and none of the other girls ever found out about her little accident. (Besides, they know she loves to pee her wetsuit anyway)

Edit: Seems any wetting that doesn't explicitly show pee is tagged "safe". To anyone who stumbles upon this pic who really dislikes this kind of thing, please don't let it affect how you see the scene, it probably didn't happen like that in the actual episode of course, besides, Rainbow was presumably able to go two days without wetting herself, a couple hours should be nothing. Just in my little mini AU, and I only intend it for people who like this kind of thing in any way.