I think depression after working hard on something is common. I know authors finishing a book often feel that way. It's a loss of a sort; there's nothing happening again yet that takes the place of all the time, effort, concentration, etc. you were putting forth.

Were I you I'd do some light reading or studying of something else; re-connect with people outside your work (so you'll have a topic to talk about with those you use to be intensely connected with). Do something good or simple for yourself; join a gym or take a cooking class, etc. and then, if nothing else, you can say "I just joined a gym; remember all those late-night pizzas we had to eat while we were working? I gained 10 pounds on them!" or something similar to get conversation going on a slightly easier/different level.

Speaking of which; as soon as I'm finished my "project" (final exam 8 May :-) I'm joining a gym :-)