I know that this is not earth shattering news, but teaching requires a lot of public speaking.
Believe it or not, even teachers get nervous sometimes. I can't even count the number of times that I have heard of teachers getting panic attacks, crying during class, or just being generally emotionally drained. Teachers are people too.
When you are growing up, it is easy to deify your teachers and think that they are some kind of super-people. They had us all fooled. I will never forget an attractive math teacher that I had in the 6th grade who told us that she didn't have any fun in college because she and her friends were "mature." Yeah right.
I consider myself a student of public speaking. I see it as an art.
One of the first things that people do when they get nervous is start talking fast. If you feel yourself getting a case of the nerves, don't panic. Don't even think it's abnormal. Take a breath, and be intentional about speaking a little more slowly. It will help you to think more clearly and move on.
It's no big deal.