Depression during pregnancy?

So before falling pregnant I was so happy since falling pregnant I seemed to have fallen into a state of depression when some days I don't even want to get out of bed, I hate going to work and I find myself making excuses to call in sick so I can stay in bed and not face anyone that day. I'm so happy with my pregnancy I love my baby all the world I have a happy healthy relationship with my partner I just can't shake the feeling of sadness for no reason some days. Has anyone else gone through this before? I feel like I'm alone as most people I know are gliding through pregnancy and absolutely loving life? Am I doing something wrong?