So I want to clarify some things.

Naruto traveled back to the past, and since he used kyuubi chakra. His little body could not stand it, and it knocked him out. He sustained damage to his head causing amnesia.

He still has the capabilities chakra and stamina of his 20 year old self, and he gets visions of what his older self has experienced, even though he doesn't technically remember them as experiences of his own, since he TECHNICALLY hasn't experienced them.

Just wanted to clear that up.