Library Books!?!?!?!?!?

Ok, so on the 1st of March I borrowed a book from my library. I then returned it really soon after, like only 2-4 days after to a lady filling in for the normal librarian. I read it so fast because it was really good. Then the library lesson, (a cl once every four weeks where you just read in the lesson) I saw the book back on the shelf and my friend started reading it. Now in April I am getting overdue notices. WTF I returned the book, but no the stupid librarian wasn’t there so he doesn’t remember it and is expecting me to return it. Anyway my friend wasn’t at school today so I couldn’t ask her about it and when I looked for the book on the shelf it wasn’t there. What should I do?