@xToNix One of my pigs is very similar. He hasn't been with me very long and despite no changes in his diet he has become a bit bloated and had weird poops on two occasions.
My vet is of the opinion that he has a delicate digestive system. After checking him thoroughly she could tell that his digestive system was having spasms and the digestive sounds were very noticeable which is unusual for a pig when seeing the vet.
We have some Metoclopromide and Metacam to use when his poops are looking funny. I'm quite experienced with pigs so my vet is okay for me to use my judgement. But if he ever becomes fully bloated or has cow pat consistency poops then he needs to do back as an emergency.
We were told that he would survive fine on a diet of pellets and hay only, as he is eating enough of them to maintain weight.
However, we can try to introduce veg very slowly to see if we can find a culprit/culprits. I'm trying that as it would be nice for him to get some veg.
We were also recommended to give him a course of Fibreplex every month/6 weeks. It won't harm but it may help stabilise his system.
Almost a year ago and a relapse 3 months later, Sooty became unable to absorb vit C and became extremely unwell. His poops became very small and stringy. But with a daily dose of 25mg of shop-bought vitamin C tablets, he got much better. The only trouble is his poops have never really returned to normal. They recently became really soft and weren’t far off diarrhoea. I stopped his veg and fed only pellets and hay for a few days which helped a little. He was neutered a few days later and the vet tested a sample which came back all clear. On a post-op check another vet said his poops looked a little dry and to reintroduce veg but the slightest bit too much causes his poops to become soft again. I also cut back on the pellets as his urine was beginning to look more powdery. So I’m at a bit of a loss which way to go with this now. I’ve read too much vitamin C can cause softer stool but with the limited veg he has he’s probably still only getting his daily requirement. When I was feeding less veg he didn’t look as swollen and he actually started doing little popcorns which I had never seen him do before. I don’t want to keep putting his tummy through too many changes and I’m also worried he’s not getting enough food. He weighed 728g yesterday which isn’t too unusual for him, he’s always been a small pig. Anyway I’ve attached a picture and would be grateful for some advice.
when you say slowly one by one would you perhaps give him some of one veg in the morning, see what his poops are like and try him with a different veg in the evening and so on? I did start a bit of a diary last night so I’ll continue with that. 
I bought some Fibreplex on Wednesday evening. I’ve been feeding him half of what I’ve been feeding the others to start with which is more than he’s had for a while and touch wood there haven’t been any cow pats. Most of his poops have been half the size they should be but there have been some normal ones amongst them so hopefully the Fiberplex is working