Given you are at 18.5 it is like you standing on the edge of a cliff and wondering why your father might be worried. He is worried, because you have proven in the past to be willing to jump and instead of being comfortably away from the edge, like a BMI of 21-22, you are standing there, toes over the edge. And then to add to your fathers pain, you look back at him and say, "Well if I get sick or have some stress, I might fall below 18.5."
To make your father feel comfortable I would recommend you take 2 easy steps...
-1- Go to a doctor and determine a minimum healthy weight for you. This will comfort your father in knowing that a 3rd person that is a doctor has helped determine a minimum healthy weight. It will also help you if your father still thinks it is not enough. You can use the doctor to calm your father and reassure him. In other words, you can use the doctors opinion to your benefit.
-2- Be open and honest with showing your father how much you weigh and that you weigh at least the minimum as agreed upon by you and the doctor. Normally I would say your weight is a personal matter and it is not for your father to be involved, but you have a history of being anorexic, so if you want to calm your father there is only one easy solution and that is open, honest communication which includes showing him how much you weigh.
Now you don't have to take my advice and make your father feel comfortable, but if you don't then I believe he will continue to insert himself and try to control you and your weight.