Why would our dog forget who we are?

We have a 2 1/2 year old English Mastiff. For the last 2 days he is acting like he doesn't know who we are. He doesnt come to us when called. And stays far away. He just stands back and looks. He will run up for just a second, like he remembers for just a moment and then runs away again. He will eat and drink though. Any ideas would be greatly appreciated.