I am very sorry for your loss. I went through the same thing after my father-in-law died in 2006. In retrospect, I believe it was a mild depression. It helped to discuss what I was feeling honestly with my mother. I also continued running regularly b/c I knew that time would help me process my feelings and help me feel "normal." I also decided to just focus on any little thing I could get done each day. I remember feeling really productive one day after just cleaning out the silverware drawer. I also remember feeling like I should have been able to snap out of it, but I couldn't. After a time, I felt "back to normal."
Above all, cut yourself some slack, and try to take care of yourself -- physically, emotionally, spiritually, whatever.