Feeling Down during Pregnancy

Is it normal to feel down all of the sudden during pregnancy? I have not been working since I got pregnant and I feel empty. Baby is coming in a months time which is a good thing but does that mean that baby is all that I have going on in my life ? Would I be able to have other topics to talk about other than baby topics? Will my sadness be passed on to my baby? Has anyone felt this way about their pregnancy? On most days I am very glad to be able to carry a baby while I read so many post about people having a hard time but some days I just feel empty.