I've read that others have had issues after fusions .

My theory - Since the spine is meant to bend and flex , fusing, it seems might cause more stresses to be transferred above and below - thus setting off other troubles.
this is just my own logic after reading many posts over the years.

Then again you might be misaligned and this is just something that happens to be surfacing now.
I don't know how you feel about expert chiropractors?
but a really good one might be of help, or an advanced PT person.