User blog comment:B214/Chapter 504: Rift/@comment-7418318-20161001212543/@comment-7418318-20161001220601

Maybe she was gawking at Natsu because she was so surprised that he could move despite the time-stop.

Or she might have suspected that Natsu would mistake Lucy for dead and let him have that moment to torment him.

Or it could be as you said.