Why do you think my puppy may be depressed acting?

I have a 5 1/2 month old Alaskan Malamute. We took him to a family member house yesterday so he could play with a year old Pit Bull. They played for hours!! My Mal is a male and the Pit is a female. Neither is neutered. My Mal doesn't play with others dogs very often because we are not around many I would consider letting him play with (Pit Bull and Rott owner that are using there dogs as guard dogs). Now today he is depressed acting and its makes me worried cause I don't want to see him that way. Could he be sore from all the playing or possibly be depressed because he made a friend and misses her?