Congratulations, you broke your cherry!
To add to what has been said, you WILL get better at this, and less nervous about speaking in real time about the law. And fast.
The getting called on in class thing has one aspect that makes it particularily impossible to deal with: the professor could care less if your answers are right or not. He has called on you merely to use you as an example of what's wrong with EVERY argument. The best you can do is smile, relax, and keep changing your mind every time the question is placed in a different perspective, as if you give a *&^%.
Really, the only way to screw up here is to not answer at all. (That typically only happens to those who are convinced that there is a right answer in the law. There is not. The law is about who argues the loudest and clearest.)