To me, it sounds like bun has a bout of gas. In your other thread, you said he was pooping. That’s really great! Baby gas drops often help with gas. They can be bought in many places like drugstores or Walmart or Tesco if you are in the UK.
Small poop, oddly shaped poop and tooth-grinding means tummy trouble, and many bunnies go through that when they molt. The important thing is that you make him move around, see to it that he eats – if he likes romaine, you can serve the leaves dripping wet so he gets water in him. Parsley, cilantro, mint and sometimes basil are also often appreciated. If the teeth-grinding goes on for more than 8 hours, I’d call a vet. They can give a painkiller and sub-cutaneous fluids. Gas is painful for rabbits and must come out.
Megacolon is sth very different, and honestly not something you need to worry about now. It’s a genetic disease and a lot rarer than gas pain/gut slowdown. It’s sth a vet would perhaps consider if a bun has frequent episodes of tummy problems and the poop looks like cow piles (it is also known as cow-pile syndrome). As LBJ also says, it’s primarily seen in rabbits with white fur and dark spots, because the gene causing it also gives rise to that color-pattern.