My dog is acting strange?

My dog is flinching whenever anyone tries to pet him. We have a pretty big pantry room and he won't go into there to go outside. He's acting like he's afraid of the pantry room and he's never done that before. There's a door in the pantry, which is my brothers room and he just ran in there and he won't come out because he's to afraid. He was acting fine about 2 or 3 hours ago. We have had him since he was a puppy and now he's about 6 years old and he's never acted like this. If anyone has any idea what could be wrong with him, any thoughts would help. Thanks