Why I'm So Nervous

Hye..
Friends, I can speak English..But why if my lecturer ask me something, i lost all my words, i will suddenly do not have anything to answer it..Why Why Why??=)
I always try speak English, not with my friends but with my blog..hehehe..
I can write easily,, but i do not know it is true or not..hehehe,,
That mean, i can speak it.. 
But, Why i always make mistake when my lecturer ask me something..
I always try to be a better in English..
But i can not speak easily with my lecturer and my friends..
Of course, at the time if i answer the question, my English will broken..hahahaha..
But it is ok...
at least i try speak at my blog..hehe
If anyone of you read my post..
please give your comment if there have a mistake in my words..hehehe..
Bye...=D