I would avoid all gluten in your food for 4 weeks. See it that changes your
bowel habits. That sandwich diet may be the problem.
Gluten intolerance can give PN symptoms.

If you have frequent diarrhea, you could also have Crohn's disease, and this needs attention. It can follow an intestinal infection from food borne illness. A colonoscopy can show large parasites, like tapeworms.
But a stool sample is needed for smaller parasites like Giardia. Sometimes the first stool sample for Giardia is normal, but since the parasite has a 3 day cycle, a second one usually shows the bug. It comes from unclean water or restaurants or contact with small children. (daycares).

Frequent loose stools leads to electrolyte disturbances, loss of potassium and magnesium. The high intake of vegetables may upset some people, due to gas formation. If they have Beano over there, I suggest you try that.

It is hard to live in a country with different food and habits. You may be intolerant of something else besides the gluten. Say soy, or the MSG that may be in the food. MSG can cause burning. Monosodium Glutamate.

We have a gluten intolerance board here. So check them out too.

The two remedies I find most useful for burning are vitamin B1 in 200mg-300mg doses/day (Thiamine) and Alka-Seltzer with aspirin (if you can take and tolerate aspirin). If you can find an ointment with menthol in it at a chemist's there, it might help to rub it on. Menthol stops burning, but does not cure it. I have no idea what is available in Korea however. If you can use the internet, Biofreeze gel is one of the best. Some countries do not import certain things however.

If you damaged your back somehow, that is always a possibility, but I would think you would have other symptoms with the back as well.

You sort of have to be a detective to ferret out your problem. I hope it resolves for you soon.