Original intent of the movie?

I watched this movie last night and came here looking for some explanation for the movie. I've seen a few people mention that it's classic post-partum depression/psychosis, and the symptoms do match. But why did she start having the symptoms before the baby was born? She saw the white mouse when they first moved into the house. The next day she went to buy the poison and the dog died. Later, when she was visiting the dog's grave and she found the doll, her water broke. Clearly, this "mouse" was already in the house before she gave birth.

As for Jenny, I keep thinking she is just the girl who lived in the house and a complete red herring. My problem with that is that when Laura called NYU, they had no record of her. Why would there be no record of her at the college?

I could be missing something. I watched the movie after the kids were in bed, but I was doing laundry, so maybe I missed something. I definitely did not see anything about her breast bleeding when she used the breast pump (FYI, my breasts bled the first time I used the breast pump. It's actually not uncommon). Did I miss something else that explains some of the loose ends?

I guess I'm wondering if this movie was supposed to go one way and then they decided to go a totally different way but didn't edit it properly.