Hello Yummy!...and Welcome to the chronic pain forum!
I'm a bit surprised that you are still on Oxycontin, however that is a choice of your surgeons. If it's for other pain problems, then yes I see that. I also have cervical issues, so even if I had a lumbar surgery, I would expect to recieve meds for the cervical problems. If...for the post lumbar surgery...then it seems to be a rare thing.
The pain you describe was the same for me, after my double lumbar fusion surgery back on 2010. I know full well what you are going through...but not sure what causes it. I can only guess though...and one thought is that it takes at least 12 months and up to 18 months for a complete fusion to take place. I had this same shock happen every once in a while clear up to the 12 months mark of recovery. For me it happened if I took a misstep or was lifting something heavy on one side of my body.
It can drop a person to their knees very quickly, and I bet you let out a hoop of a holler too!
When is your next checkup?...I would mention it then and see if they have an answer for you.
Take care, and again!...welcome to the forum
SE