My kitten won't poop!!!!?

I found a stray little Calico kitten the day before yesterday, I was told she is about 6 weeks, I nursed her with a baby bottle and some kitten formula the first day and then gave her some wet kitten food since (which she loves), she even learned to use the littler box right away but she won't poop! Her tummy feels full and hard and I have a vet appointment on Tuesday but I am worried sick she will die because she will not go number two, I did everything from moist Cotton ball to her little tush to sitting her in warm water, she just refuses to go, any ideas?????