iPhone 5s flex on the left edge

I discovered this accidentally yesterday after 3 months of having an iPhone 5s: The left side of the phone - there are a few spots that are "flexed" a bit, meaning when my finguer press the screen, I can hear very minor sound from the hardware flex. It is not "solid".

There are two spots like that. I don't have that when it is new and there are two spots have this issue. Sometimes it will flex a while, go away, and come back after a while.

I brought to Apple store. They made a Genius Bar appointment. Anyone having the similar problem?