Welcome daoisthere,,
Sorry for your diagnosis and know this can be an unnerving time for you,,, relax and try to research on here as there are many of us that post and have been in your situation,,,
I might venture to say that if the MRI for the concussion was done without the contrast dye, it may not have found a small AN ,, so it is possible it existed prior to the last MRI,,, just a thought,,, you don't say why your last MRI was done,,,,
I would just recommend discussing with your doctor the next step and perhaps getting a second or/and third opinion,, it appears to be rather small with few debilitating symptoms at this point so further testing and observation may be prudent,,,
Good luck to you,, as most of us will tell you, there is no " normal" case of these tumors,, everyone is different in their desires to treat or not treat,, and their symptoms,,and reactions to having a tumor,,,
We are here to help and most are not medical professionals,, so our advice is given as just that,, advice from our personal experiences,,,
Jane