I am wondering what kind of pain you are having. You mentioned back spasms. Did you have them before surgery? That's a different kind of pain than the normal post-surgical kind of thing.
My onc told me that gapapentin helps some people with neuropathy. My PCP prescribed it for me in hopes of helping with my Keytruda-caused arthralgia, but it did nothing. Gabapentin is primarily effective for nerve pain. Flexeril is a muscle relaxant that blocks nerve impulses of pain sensation sent to the brain. You aren't suppose to take it with opioids. It sounds as if they are concentrating on your back pain. Maybe because they can't up your dose of oxycodone without you going to the ER.
I distinctly recall having the option of taking 10mg of oxycodone instead of 5 if necessary for a short period after I was off the epidural, and it definitely made a difference and worked well. But back spasms were not the source of my pain.
When I DID have back spasm-type pain for a while many years ago, the best advice I got (from a physical therapist) was to take the pain killer (ibuprofen)on a strict schedule every 3 hours for IIRC 3 days and not wait to feel it. The idea was to break the cycle of pain, and it worked. The pain was gone completely after the 3 days, after having plagued me for a few weeks. Note that I am not suggesting that YOU do this!
I think it is important to understand why they are prescribing those particular meds, and exactly how to take them for maximum benefit. If they didn't explain, call and ask.
F, 64 at DX CRC Stage IV
3/17/18 blockage, r hemi
11 of 25 LN,5 mesentery nodes
5mm liver met
pT3 pN2b pM1
BRAF wild, KRAS G12D
7/18 and 11/18 CT NED
12/18 MRI 5mm liver mass, 2 LNs in porta hepatis
6/19 Multiphasic CT LNs normal, Liver stable
6/28/19 Pause Key, predisone for joint pain
7/31/19 Restart Key
9/19 CT stable
Pain: all fails but Celebrex
12/23/19 CT stable
5/19 MRI stable/NED
All MRIs NED