I've strangely found myself at gigs (and concerts back in my cello playing days) thinking random thoughts in the middle of a song like "Did I lock my front door?" WTF is all that about?
I digress, nerves are a funny thing and affect people in different ways. The not being able to recall what comes after the bit you are currently playing is quite common. The extra adrenaline that nerves serve up can make your grey matter operate at a higher/different speed/plane/rate, whatever you want to call it than in a comfortable rehearsal environment. As you say, you do actually remember and play the bit that comes after. This happens to me too occasionally, but I know I'll remember it and that helps relax me and lo...I do remember it.
Looking away from a double page of sheet music in a concert situation for a second then looking back in the wrong place used to panic me more!