Is this a one week old kitten or a kitten you've had one week? Regardless, never bath a kitten in water unless it's an emergency, use pet wipes instead.
I need a bit more detail on it's age (a one week old needs very specific, careful care), but young kittens tend to have distended bellies. Unless your kitten seems sick, it's probably just the normal course of growing.
Is it correct that a week old kitten should be given water, not milk? I was told that, when we took in a stray kitten.
Never milk. cats are lactose intolerant, it will cause vomiting and diarrhea.
What's best is a milk replacer, which is mixed with water. I use KMR Milk Replacer...1st stage for a kitten that young. If you can't get that one the pet store will likely have one just as good.
One thing to remember with a kitten that young is they can't urinate without help, so it's best to take a cloth damped with warm water and gently rub "the appropriate area" to induce urinating (the idea is to mimic the mother cat's licking motion which would normally take care of this).