It may be hormones, or just stress. It is easy to act all happy about it, because once you are pregnant what else is there really? She may still wish that she wasn't pregnant (which is totally normal and understandable). Maybe she just wasn't ready yet to be pregnant, or maybe she doesn't want to be pregnant at all.
People try to act like once you are pregnant it is all laughs and smiles and excitement, but to a lot of people it isn't, and that is totally normal!
She will come around, hun. Once it all sets in and maybe after she sees the baby (ultrasound, etc. ) she will be happier about it. I have a friend who just had her baby and she was miserable throughout the whole pregnancy. She tried to act happy, but she just didn't want a baby. Now that the baby is here she is just beside herself with joy.
But, I have another friend, same situation, but even though her kids are 8 and 9 now still says if she had to do it over again, even though she loves her kids more than anything in the whole world, she wouldn't have kids. Not every woman is meant to reproduce and that doesn't make her defective or anything.
But, anyway, back to your situation, she has you to be there for her and you seem happy about this baby, so I think you two will be fine. Just be there for her and help her through this and I think it will all work out.