Hello, and welcome to Psych Central! I had some trouble with postpartum depression with my first, but I just felt anxious for some reason during all my second pregnancy. I wasn't clinically depressed or anything, but I just wasn't as happy either as with the first one and felt more miserable over all.
When I saw the title of your post, my first thought was "Why should she be happy?" Not all pregnancies are planned, the mom is possibly frazzled with dealing with the first child, etc.
Please try not to feel guilty. You are doing all you can with the meds.