Best way for us to tell if your animal appears healthy is to post a pic of your animal and your setup. At three years of age her weight is very low. Medium rats are a little large for that weight but if she is at three feet and can keep them down it is fine. The good news is her growth is not stunted. If she is otherwise healthy and her environment is good she will do fine. I would predict quite a bit of growth especially weight wise over the next couple months.
Make sure you take a look at this care sheet.