That's a rough one, especially since you're living with her. If you don't think (or have already tried) sitting down and talking with her will work, then perhaps you need to compile a bit of research for her on the calories you're taking in, and the nutrient goals that you're reaching. I know that she's just worried about you and obviously loves you, but maybe if she sees the facts of how what you're doing is promoting a healthy lifestyle, it will start to sink in.
Something you need to make sure of is that you're reminding yourself of the awesome accomplishment you've already achieved and of how committed you have been and are determined to be healthy. Be proud of yourself! Hopefully your mom will come around and see how positive of an effect this change is having on you...just remember though, Moms have two duties in life: to love us, and to nag the heck out of us. :P