Is there an explanation for sudden memory loss?!

I didn't know any other way to "word" this question, but here is what happened. Me and my boyfriend were walking and all the sudden he drops my hand and holds his head. I called his name to make sure he was okay and he asked, "who?". Then he drops his hands, looks up at me says, "I forgot who you were for a second." So it appears like he lost his memory and gained it back within seconds but I don't have any explanation for this. Could you tell me anything about this? A name for it, maybe?