Well for one thing, your body does get used to having the amount of calories a day you allow it to have, but pretty much every functional body wants a certain amount which is why generally metabolisms will slow down when the body isn't meeting that requirement. Starving yourself is always going to make you metabolism rebel. The best option is to do exercise. It's a beautiful thing really, will make you feel good, use calories, etc. It's way healthier and has a ton of health benefits. So instead of starving yourself try making good food choices, and exercising instead with a normal amount of calories to keep your metabolism going in top top shape.
Also, a week isn't generally a good measure of weight loss or gain. I know TV shows show such drastic changes in weight every week for contestants but unless you're like horridly obese and working out like crazy, you aren't going to see numbers like that pop up. A week isn't a time to tell, and starving yourself isn't the solution (trust me, I tried it).
Also, look into this http://www.endocrineweb.com/hypo1.html
I highly doubt it pertains to you and you hopefully are not a hypochondriac who will just believe you have hypothyroidism, but it is something to consider in case there really is something wrong with your body.
Honestly, I bet your body is fine. Starving yourself will work in one thing-controlling you (or killing you). Really, that's it. Take my word and eat well and exercise to lose weight.
Probably a waste of a post, I remember when i was anorexic. It's the funniest thing to ask for help just so you can say you did even though you don't want it and don't listen normally. Ah well.
"My open-ness is how I lie, my submission is how I control."
None of us are virgins, life has screwed us all.
Sometimes you have to fall in order to see who will be there to catch you.
No,that doesn't mean I'm gay and in denial.
No, that doesn't mean I'm straight and horny as hell.
Yes, that does mean I'm twice as likely to have a date this weekend. =P