Super depressed anyone else?

I'm not sad that I'm pregnant, I want this baby. I want to love it and cuddle it and all the things moms want. But I just can't shake this sad feeling, I have two other kids and I'm trying to just enjoy them and not stress or worry about the outcome of this pregnancy and I feel like I'm just not all here. I have an Appt next week and I'm going to talk to my doctor about it, I just don't know what's going on with me. And I feel terrible for being sad during such an exciting time.