My Chihuahua Was Walking Around Like He Usually Does And

My chihuahua was walking around like he usually does and out of the blue he starts screaming... I picked him up to see what was wrong and he acted very frightened like he didn't know where he was for about almost 2 minutes. Then started coming around and looking around. Like he didn't know what happened. He did not shake or convulse at all. What could've caused this? Now he's laying down looking worried and tired. He's 6 years old and this has never happened before. He's always alert and happy.