Lets not scare her with Arachnoiditis. I dont think the symptoms match that of Arachnoiditis. I think like others have stated, she's looking more at mechanical pain and/or inflammatory pain.
It takes the nerves a long time to heal, and they heal at an average rate of one inch per month -- and it can be painful during the process!!! There's been a lot of cutting thru nerves/etc. and now they're trying to heal. Plus there's still alot of inflammation in there as you're still pretty early in the healing process.
I wasn't able to go back to work part time until 4 months after surgery, and not full time until 6 months!! And I didn't have FUSION. I just had a laminectomy/diskectomy/foramenotomy. So you had alot more done that I did.
I wish you the very best. God bless and please take care. Hugs, Lee