Feels like you nailed it! Well done!
This was my first thought as well. My daughter had this syndrome when she was in her teens, it seemed to come about during her monthly cycle. It has subsided as she has entered adulthood. The lesions were restricted to her tonsils but with the high fever It mimicked strep and she always tested negative. She's 26 now and hasn't had enough episodes for at least 7 years.