What is causing our guinea pig's change in feces?

He was taken to the vet yesterday for eating and drinking less, and showing signs of weakness and fatigue. He had x-rays and no extreme bloat and no blockage was found, though he was dehydrated. He was pumped with critical care by the vet and then by us, and has started eating some pellets, hay, and green pepper again. His feces are coming out connected in either a long "string" pattern or clumps, with some normal. I wouldn't call it "diahrea" because it isn't that loose, it is just moist and softer than usual. Could this be due to having so much critical care without solid food? He is acting about 80% back to himself today, compared to about 30% yesterday, but this feces issue is still concerning me. Any advice or personal stories are appreciated.