Sprites reshape unexpectedly

Using Unity 4.6.0. I import some sprites to my project. there's a picture of one of these sprites (It's just a prefab with SpriteRenderer Component attached to it)

But sometimes Unity reshape the sprite automatically

And if I change some settings of these sprites like Max Size or Format, then the sprites will be rendered correctly but after a while it will be reshaped again! I don't have any idea about this problem.