You made a mistake somewhere. So either there is a bug in your code, or you misread some help, etc. Who knows? Computers do exactly as they are told to do. You told your computer to do something the wrong way, on way or another.
Given the incredibly vague information you have provided, it is very difficult to know more. The crystal ball is so foggy today. I sent away for some Murphy's Magic Crystal Ball Cleaner via mail order though, so if you wait a few days, perhaps it will arrive.
If you want a better response, then show what you have done. Perhaps attaching the model you have fit as a .mat file would be useful. Show the plot you generated that you think seems wrong. Showing your code would be very useful too. Otherwise, you cannot expect much more than vague generalities.
