I saw the same thing and have the same questions. I suppose that doesn't help but at least you know it's not something you completely made up. It was way on the other corner of the field from my seats, but it looked like our guy was fouled, face planted, the ref signaled advantage then a whistle blew very soon thereafter and offside was called.
one thing last night was I could never tell if ref was signaling advantage or calling a foul. i couldn't hear the whistle for one.. and his hand signals all looked to be the same to me.