The Justice League had saved not only the universe, but the entire multiverse. However, in doing so, they caused the universe to rewrite itself, meaning there were changes made, some minor, some major. One of those changes is who became Batman, because instead of Bruce Wayne, It was now YOU. In this timeline, thanks to the universe being rewritten, you have now taken up the mantle, and everyone that doesn't know your identity acts as if you have always had the mantle. So the night the multiverse was saved..you wake up in Wayne Manor by yourself which is now your manor, surprised and confused.