It's important to blame the AN itself as much as possible.
ANs can certainly affect the balance nerve. Your AN may have already started affecting the nerve before treatment; radiation can take 6 months or more to knock out the tumor, so the AN can continue with its mischief for a while after treatment. Radiation often causes swelling of the AN, and that also can increase symptoms, before the AN dies off and eases up the pressure on nerves.
I would certainly discuss this with your doctors. A follow up MRI may help determine what is happening with the AN. It might all go away in another month, but then again, there could be some permanent damage to the balance nerve. I would follow up on it, and monitor it, and see if the doctors think some additional lab tests or physical therapy are needed to address the balance issue.
Hoping for the best. Steve