Hello~I would call your prescribing physician and ask what is going on. Maybe they just want to check some other issues. I think if it was serious, they would call, sending a letter means they are not that worried.
Another thought that comes to mind, they have sent the letter to the wrong patient, it may not have been meant for you at all, another reason, therefore, to call and see what is going on.
As someone in the medical profession, I can not tell what is going on. Nor can anyone else. The best course of action is to call the doctor that the letter came from and ask them about it. I am of this thought, if it is serious they would call, if it's something that isn't that serious then a letter is sent. Hope all is well! :)