Hi and welcome to the Chiari forum.
There are several things that could be at play causing your pain....one did you do neck exercises post op and if so, are you still doing them? Next, do you have any other related condition? If so, what are they?
It can be related to surgery but with out more info it would be difficult to say....
What all was done during your decompression? Laminectomy. dura plasty? If dura plasty what type of patch?