Feline constipation, gas, possible FIC seemingly resolved, but still in pain/hiding.

My cat (8M, neutered) has been perfectly healthy up until a few weeks ago when I changed his food to a ‘Sensitive Stomach’ variety of the same brand as he gets hairballs and sometimes eats too fast and throws up. A few days later, he started acting strange (i.e., hiding, growing when touched, lying in a hunched position, hard and slightly distended abdomen). I took him to the emergency vet where he was x-rayed and treated for constipation with an enema, pain medication, and IV fluids. His bladder was somewhat full, but not concerning. He was isolated from my other cat for the night to ensure he urinated. He went through a couple of hours of going in and out of the litter box, straining.

Unsure of if this were due to bladder issues and knowing how quickly they can be dangerous, I took him back to the emergency vet. They gave him a catheter, kept him for two nights, then tested the urine, and performed an ultrasound. Everything came back looking fine (no urinary crystals, grit, stones, or infection). He didn’t appear to have any intestinal issues and all bloodwork came back clear as well. We chocked it up to possible FIC due to stress (recently moving) or constipation and gas due to changing food. The vet recommended a urinary diet (Hills Science) and MiraLAX, so his diet changed again.

He appeared completely fine for a week or so, until he slowly started acting more clingy, sitting in a loaded position more often, and not jumping in bed. We restarted MiraLAX in his food for two days. The next day his stomach returned back to hard and slightly distended and he started hiding all day and night under the bed. He was still eating, drinking, and peeing, but we took him back to the emergency vet. They took another x-ray and determined that his bladder was small and not a problem, but that he was moderately constipated and showed a fair amount of gas. They told us the MiraLAX could be causing the gas and to discontinue it, but continue pain meds. He was given pain medication, subcutaneous fluids, and sent home as he was still eating, drinking, and peeing.

At this point, I felt the food must have been the cause and went back to his original food. He finally pooped about a day later and we expected his pain to subside soon after. It has been six days since then and he is continuing to hide all day and night, yelp/hiss in pain when lifted, stay in the loaf position, and unable to jump onto the bed. I called the emergency vet and his primary vet yesterday to get their input and both said as long as he is eating, drinking, peeing, and pooping that it’s not an emergency, but that we could bring him in Tuesday when there are openings if he doesn’t improve.

I know my cat, he’s very forgiving and flexible. He doesn’t hold grudges, isn’t anti-social, and wouldn’t growl or hiss unless he had a really good reason. He is normally so affectionate and sleeps in my bed every night, so this is very concerning. Does anyone know what else this could be? I’m concerned about an infection of some sort, or some other underlying issue that is being overlooked. I’m so incredibly worried and appreciate any and all input. Thank you!

TL;DR: 8-y/o M cat experienced two bouts of constipation over the course of a few weeks. Has recovered from constipation, but still showing signs of serious pain for almost a week. Anything else we should explore diagnostically?