It sounds like she is nesting. Maybe when she begins pushing she will go into her nest. If not, put towels in your lap and put the kittens in the nest as she has them.
The nesting box does need to be large enough for mom to stretch out in when nursing.
The fist stage took ages for sula. I even started to get worried. Is she contracting? Sulas was very similar to ours they were far apart and weak for ages. She was still even eating and walking around. Then she started staying in the nest more and they sped up and got strop. She won't stop crting for me now