Breech kittens are not uncommon in cats. If the kittens seems stuck (if she is pushing and the kitten isn't progressing) you may have to help out. Gently wrap a cloth around the kitten's legs and give a gentle tug in time with her next contraction. Kittens usually pop out within a few minutes of their first body part showing.
Is she still pushing but the kitten isn't progressing? Does it seem stuck? If it does, rush her to the vet clinic. The kittens may be big as they 'cooked' for so long.
Don't let her walk around. Take her into a room and stay there with her. It's fine if she needs to stand up, sit down, move a little, stand up, sit down etc but she needs to be confined. Try putting her in the nest that you've prepared.