My dog has been tired lately.

My normally super-energetic dog has been pretty tired lately. We've just moved into our new house three days ago, and all he's done all day is sleep on my bed. We've taken him to the house a few times before we moved in so he could get a feel of the place, and he really liked it there (he didn't want to leave the first time we took him over there). I'm really worried because this is completely out of character for him. Any ideas as to what's going on?