OMG : What should i do ?

Well first of all, I think you shouldn't have done that, but now that you have done it, just hope your teacher doesn't notice. However, if she does notice, then just say it was a little, careless mistake and that you're really sorry. Try to look really sincere. But you know, it would be better if you just tell the truth because you don't want to get into further trouble. Good luck for whatever happens!