Hi and welcome to the Chiari forum.
May I ask, did they rule out co-morbid conditions of Chiari prior to your surgery?
Over the years I have learned there are no pain meds to help with Chiari pain....but it can help to know what is actually the root of the pain, Chiari can affect us in many ways and bring along other conditions, so you need to find a Dr that looks for these related conditions, as it could be one of those causing your issues OR you could have issues due to the surgery itself....scar tissue can form and cause issues....
Have you had follow up MRI's?