2019 Subaru Impreza

I have a ‘19 Subaru Impreza, sometimes my rpm will go up to 2000 and will not move when I accelerate or slow down. Example: I sped up to 75mph and slammed on my brakes at 50 mph and the rpm still did not move. This doesn’t happen all the time. It will go away while driving and go back to normal. Is this the transmission fluid? Any advice will be helpful!