During chapter 344 Erza is regressed to a child like form due to a magic called Law of Retrogression and later is forced to face off against Minerva, later in chapter 349 Gray causes the one responsible to lose his focus on the magic and Erza returns to normal just in time to save herself from being cut down.
http://mangafox.me/manga/fairy_tail/vTBD/c349/16.html