When I was about 12 years old, I read a children's book (aimed at age 8+ I think) that was a fictional diary of a young girl that lived in the Middle Ages. She was quite a rebel and wrote the diary in a very funny matter. I remember laughing very hard when her parents had decided that she had to marry the son of another nobleman. She didn't agree with that and when her fiancee came to visit, she set the toilet on fire whilst he was sitting on it. That's about all the details I remember from this book unfortunately.
Does anyone here have a clue what the title might be or the author?