As has been mentioned, you could possibly be feeling that depression because of the loss of this project--yes, you completed it well and it went over wonderfully, but it's still OVER. It's still a loss, and part of you is feeling that.
Let me put it this way: whenever my boyfriend finishes a show (he's an actor), he always goes into "withdrawal", so to speak, and is often a bit depressed. It's worse if the show and the experiences he had leading up to it (rehearsals, cast dinners, etc.) were really great. Keep in mind that he's 100% normal psychologically, so perhaps you're experiencing something similar, but your prior depression is making it worse?
Also, as to having trouble relating to other people outside of work--perhaps by becoming very comfortable and familiar with the people you work with and the social structure of that environment, you find it hard to relate to others becuase they're not a part of this intense project you've poured so much of yourself in.
Hope this helps,
luck&love,
~muse