Hi and welcome to the Chiari forum.
There are several related conditions and a few things that could have occurred due to the surgery itself....Related conditions would be Syringomyelia , POTS, ICP, CCI, EDS,retroflexed odontoid, basilar ingvagnation, ......
Surgery issues could be cerebral ptosis.....also known as brain slump....this is when too much bone is removed and your brain is falling down further then before and could be compressing the brain stem and or spinal cord.
If you have EDS and had surgery you could be rejecting the dura patch if one was placed that was not made of your own tissues.....