Cat delivered a stillborn kitten at 11pm, no more contractions, still big belly

From your description, the cat delivered a stillborn and still has a large belly, indicating dystocia with likely retained fetuses. Take it to a vet promptly. Usually oxytocin is given to pass the stillborn; if that fails, surgery is needed, since a dissolved fetus in the uterus can cause pyometra and permanent uterine damage.