Try to explain by lying
From Create Your Own Story
-You then right away tell her not to say anything to anybody. After closing the door of your room, you both sit on the bed and you start your fake story. You tell your mom that you never had any kind of sexual experiences with someone else and that you wanted to see if your thing is actually big or not. It took you over 20 minutes to convince her that your story is true. After a couple of question, she finally decides that she wont tell anything about this to anyone. (great you are out of trouble)
Then you:
