Stray pregnant cat still hasnt had her kittens!?

I found a stray female cat about 12 weeks ago, and we decided to keep her. Her stomach started to get bigger, so we gathered she was pregnant. 9 weeks pasted and still no babies so we took her to the vet and they said that there 99% sure that shes pregnant and is around half way through her pregnancy, another 4 weeks has pasted now and still no sign of the kittens! im so confused is she pregnant or not? she has all the symptoms, her stomach is rather large and solid, and she has very large pink nipples, she also started nesting for about 2 weeks and then stopped. What do you think has happened? is it possible that they have died inside her? shes still eating and drinking normally.