Something Weird Happend To My Friend! He Can't Remember A Ceratin Day?

Okay, so a while ago (over a year) me and my friend went on this hike to the lake, and we were talking about it today on Facebook, and he says he doesn't remember that day AT ALL! Not even bits and pieces. I even send him pictures to jog his memory and he still says he doesn't remember. There was something weird about him that day. He was acting weird, saying weird stuff, he wasn't himself. I know it was a while ago, but he should at least remember bits and pieces if it. We did get home pretty late that day. It was already dark when we got back. IDK. Could this be some sort of short term memory loss or amnesia? I didn't think much of it at first, I thought he just forgot about the boat we were gonna swim to, but then he said he doesn't remember anything that happend that day! I asked him like 3 times if he's messing with me, and he said no, he really doesn't remember. Something weird is going on here! What could this be? Could that happend to someone who was drunk or on medication? 'Cause like I said, he wasn't acting like himself AT ALL! And he has no twins that either of us know about!