If I remember right, Mike Tyson admitted something similar. He got so nervous before his fights that he occasionally even had to vomit, but then once stepped into the ring he switched on the fight mode.
Very true. I've also heard GSP talk about how scared he is come fight time. I'll try and dig up the interview. He said the last thing he thinks before entering the cage is, "what the hell are you doing here".
It seems a common thing with fighters. Its something I love as a fan when they are honest about their feelings.