Post Partum Depression... Is it possible?

I'm 23 weeks pregnant and feel like I'm slipping into that depression phase. I had it bad after I delivered my first. I was on medication for almost two years. I'm constantly stressed. My best friend is moving to a different city to be closer to her girl friends. She currently lives ten minutes from me. I even told her today to not worry about throwing me a baby shower... I just feel like crawling into a hole. I'm beyond excited for this child because we spent three years trying to conceive. I just don't know what is going on.