After rereading your post I checked you BMI and you are a little underweight, so I now understand your parents concern. 110 lbs is more like it for your height.

So here is a healthy idea- just agree with them and tell them you'll gain weight. BUT compromise with them and tell them your concerns of about gaining fat. So tell them you are going to gain lean mass instead and just do it all X style. Pick up some creatine, a quality weight gainer, up the cals to 2000+ of clean calories. Couple that with P90X classic and you'll gain muscle, muscle, muscle. Creatine alone with pack on 5-10 lbs. If you do this you might change their perspective on beachbody programs. Show them it's not all about "diet" and more about the healthy lifestyle.

Ohh yea I remember that thread. You looked good before and after personally I agree with Yamar. Sit down with your mother and go over everything and see if you can come up with a comprimise. You have some very nice tone and it would be a shame if you lost that but you should put on a little extra weight. Hopefully she will be flexible.

You are a normal, working adult. Interacting with your parents in this way is extremely unhealthy. No one can "force" you to make specific personal lifestyle changes. When someone is concerned about you, it does not "indicate that" or "follow that" you need to do what they want, when they want it.

The complete lack of a sense of personal boundary and personal autonomy in your posts is very disturbing. It sounds like there may have been a long history of controlling/demanding behavior and denial of acceptance/approval by your mother, but at this point, you are in control of your own life and you have only yourself to blame for choosing to continue the cycle of harmful behavior.

This is a sad state of affairs. After eating healthy foods why would one turn to cookies and pies just to gain FAT back?

You don't look scrawny you actually have toned muscles and a wash board stomach.

Oh and by the way, I know that working 15 hours and then doing P90X is extremely hard so congratulations on that.

BMI is useless. In his prime, someone like Arnold Schwatrzanegger would have been classed as obese by using BMI. It does not consider body composition at all (and niether do you parents, apparently).
Your Mom's a dietician? No offense, but it sounds like she needs to get a new career. Trying to force Doritos and fries on you and placing a moratorium on excercise is insane. Are they pushing booze, smokes, and IV drugs on you too?
Speaking of Insanity, having done it, I say you challenge your parents to keep up with you for even 5 minutes (let alone 50) on one of those workouts and then tell you you're unhealthy.
Your parents should read this board and this thread in particular.
At the same time you need to stand up for yourself. You're going to grad school but Mommy and Daddy and still packing your lunches and telling you when it's time to come in from playing outside (essentially).
All that said I can understand how your transformation would have shocked them if all they saw was you "before" and then you "after" months later. I have no doubt that they are doing what they are doing out of concern for thier daughter. Just going about it the worng way. Maybe it's just a matter of educating them a bit as to what these programs are all about and how they have helped you.