It happens to all of us, last time I was interviewing I was asked the exact same question two days in a row, the first time I answered it without trouble, but the very next day I choked. It wasn't even a difficult question. After that the interview went downhill.

I was pretty discouraged after that, but all you can do is go on to the next one with an open mind. There are lots of great opportunities out there. A lot of interviewers understand nerves and will take that into account