Got accused of plagiarizing on Midterm and Final exam! Help?

I can understand how something like that could happen, but if you were accused of this on your midterm then why didn't you clarify things before your final exam? Unless your teacher waited until the end of the semester to grade the mid-term, it seems to me that there really isn't any excuse for this happening on both. I'm really sorry, but you should have known better. I do hope that your teacher will work with you and that you don't get into serious academic trouble over it.