Pregnancy Depression

Found out the gender yesterday and I was so excited that I'm going to have a baby girl. But last night I got hit with this wave of depression and it's worrying me. I have had depression before I got pregnant and I don't think it's ever been this bad. I feel so depressed about having this baby, that I planned. I planned this pregnancy and to have this baby and I'm depressed about it?? The thought of raising and taking care of a child has always made me so happy. So why is it now that I had the most exciting ultrasound of this pregnancy, I got hit with this depression? I'm just worried I'm not a good mother or won't be because of how I'm feeling. I don't know, it's nothing I can't handle I just don't know why I'm feeling like this. Any help would be appreciated.