If you had a concussion and can't remember anything, does that mean that something is wrong with your brain?

For instance,after you had a concussion, if the doctor gave you tests on your memory, thinking, balance, reaction time, etc., and your results weren't normal or up to standards, would that mean that there is DEFINITELY something wrong in your brain like a bruised vessel or an injured nerve or something? Or can you fail some of the tests that the doctor gives you and they find nothing wrong with your brain when they give you like a CAT scan or whatever it's called? And also, when you hit your head and pass out from it, does it mean that something has CERTAINLY been injured in yoyr brain, or could it have been a fluke and it just hit your skull or something?