Odd doggy

My sister's dog bit me today. He's never done that before. He was lying in the doorway to her bedroom looking adorable as always, so I strolled over and bent down to softly scratch his ears...after a few seconds he snarled and chomped down on my arm! He didn't draw blood, but his teeth did leave marks and a few scrapes. I have no idea what set him off.....he's standing next to me with his head in my lap right now, sweet as ever, soaking up the petting. Perhaps he was defending his master's territory....or maybe he didn't recognize me at first (I had just come from the shower after using a new shampoo). Who knows?