User blog comment:FairyRaven/Fairy Tail Chapter 351: Eternal Fire, Review and Commentary/@comment-5426822-20130912215011/@comment-7418318-20130912221946

Considering that Wendy stood right next to the Eclipse Gate when the Dragons came through, Atlas Flame must have passed right in front of her on his way into the city, so she'd probably got to feel the texture (or whatever you want to call it) of his magic for a bit, before the next Dragon paraded past.

But I don't think that Gray and Wendy were feeling the same magic. Gray was feeling magic from the ice itself, while Wendy is now feeling something approaching. I suspect this something was encased in ice and was now set free by Gray.

And judging from Wendy's expression it's nothing good....

Come to think of it, does Wendy know about Atlas Flame becoming an ally mid-fight? Because if she never learned of this and still thought he was an enemy, it would make sense for her to be afraid, if she were to feel his presence.

Or the "Gate to the Underworld/Netherworld/Hades" (pick your favourite translation...) that Doriarte mentioned is finally opening for real and Tartaros comes storming out in full force.