I think it may just be the difference between fantasy and reality.
The idea of having kids can be very exciting, but it can also be scary as well! It's definitely ok for you to have mixed feelings on the matter. I agree with Kathy that it is very important for you to discuss this with your husband. If working is important to you then your partner should be open to discussing those possibilities with you.
You certainly aren't the only woman to go through these feelings either. While I personally do not have a history of looking forward to motherhood, I do have very strong feelings on wanting to continue work even after my partner and I begin a family (if we do). Working can be a big part of your identity, and it has been a large part of mine for over a decade, that's difficult to adjust to!
So when you're happy (Hurray!), or sad (Aw!),
Or frightened (Eeek!), or mad (Rats!)
An interjection starts a sentence right.