S.L.
My family loves "Everyone Poops" also. It's a cute little book. My advice is to a) recognize that this is normal and she'll get through it in time and b) help her out by being understanding. You're already doing that some by saying "it's ok to go poo-pee" but you can take that one step further by validating her fears or her discomfort. You can say, "It feels funny doesn't it? Does it scare you? Daddy and I get that feeling when we have to go too. It's good because it tells us when to go to the toilet. It means that you're getting to be a big girl and when you're ready you can poop in the potty just like Daddy and I and all the big kids that you know."
This way you give it a spin in a positive direction. You let her know that it's normal but it IS kinda weird and it's ok for her to be a little scared, but she'll get through it. And you kind of congratulate her on getting older and more mature. Don't expect her to change anything right away, but keep giving her that message that you know she'll be able to go poop in the potty when she's ready and that you'll be ready when she is.