I have heard this from a few people, if you google your problem you will see that you are not alone.

Since it is on the left side and happens after eating that means your stomach is putting pressure on the nerve. Stomach is on left side of the body so that makes sense.

There are a few things I have read. One, people with reflux and stomach ulcers can have this issue. Two, possibly it is some misalignment in your lower spine and pelvic bones. Three, possibly there is some inflammation on the left side of the lower back, usually internal.

Things that can help you are, of course, see a chiropractor. Get adjustment of spine and pelvic. Eat frequent small meals. Include cayenne pepper and turmeric in your foods. There are many videos on YouTube for simple exercise that may help. Hopefully you are still eating plant based foods.

Lastly, if nothing helps then just fast for two to three days and see any correction happens. If not, longer fast may be necessary.

Please keep us posted so we all can learn.

Archus

Agreed with Arcus and InCharge. You certainly have some misalignment/pinching of nerves taht makes them more sensitive to other influences; I would also bet you have food intolerances . In the case of wheat, even once a year can induce clinical or subclinical inflammation lasting for months in a row. Lab testings are not reliable, since even advanced tests would take into account about 15 compounds of the about 10 thousand immunogenic fragments that Gluten gives in the course of digestion. Further Gluten is present also in Rye, Barley and Oat, and if this wasn't enough, many more grains have Glutenous substances. So that the "Wheat belly" syndrome is more like the "Grain belly" Syndrome. And it does not need to come with overt GIT distress, it may even show up as a neuropathic pain alone, as in your case.
Conclusion: It would make sense to exclude grain for a year, in order for you to be convinced it is playing your nerves - pun intended.

A lot of back pain comes from a congested colon. I used Schulze's formulas for cleaning out the colon.

Next, find a good chiropractor with a Thompson table who knows how to do traction and correct Cervical one and two. You might get some exercises that would help both the lower back curve and higher neck curve stay adjusted. They are both working together. Chiropractors have more answers to structural problems as well as diseases than you know.

have you tried to strengthen your back and pelvis with exerices?