It could be many things from constipation to FIP or anything in between. Please, take your kitten to the vet immediately. Don't wait for tomorrow, get her to an ER center or whoever is on call, right now.
PS is she spayed? If not it may be her heat cycle, but bloating is not part of heat cycle. Bloating is an indication that something is wrong, seriously wrong, which is why I urged you to take action now.