There could be any number of causes, so I don't really think anyone's guessing about any of that would be a good idea.
I do know one thing, though, and that's the if a cat is "irregular" it may help to take him/her off dry food for awhile if that's what they eat. A search will show you that constipation could be one cause. I also know (from the long-haired cat that my son had) that long-haired cats can, for some reason, have a little more trouble with remaining sufficiently hydrated (which then can contribute to irregularity). One vet, in one online article, said that in years of being a vet she had only seen two cases of constipation in cats that ate a canned or home-made diet. (Jean Hofve, DVM).
Cats are known to be more prone to having trouble digesting milk, so bloating can be one of the results. Some cats do better with milk than others. Expectant-mother cats' abdomens can feel hard, but there are a lot of medical conditions that can cause it too. Hairballs and bones can cause obstructions, so waiting too long to see a vet isn't good (and I imagine you already know that anyway).