You are not alone. This has happened to many others. My first thought is would your last doc write a letter for you explaining the larger dose and why he/she prescribed it all that time? Absent that, get your medical records from your last doc and share them with your new doctor. There should be adequate notes and prescription history there.
ADDitude Community Moderator, Parenting ADHD Coach, Podcaster & Author, Mom to teen w/ ADHD, LDs, and autism