^ Seems Rey surprised her good self that she actually managed to lift the rocks - and that the attempt to do so did in fact require a focus of effort and concentration.
There is seemingly also a sense of wonder there too in Rey using the force for this task - Rey knows how to perform such a task from her learning various abilities during her interrogation by Kylo Ren. Yet she is unsure she can actually perform, or execute it, given her lack of experience or practice - and the situation she and her friends finds themselves in. To me, that sense of wonder, actually elevates the scene (no pun intended 😉)
For me, it is also a marker in her development - one where she can take confidence and belief from - on her journey / learning of her own ability to use the force.