Im on Phentermine for weightloss....how many calories should I eat?

Ok, before I get a bunch of people bashing my diet...I am 30 years old, I have 3 kids, I'm 5'4" and I weigh 205 pounds and I wear size 18. I used to be 135 and in size 7-8. My Dr. has prescribed me this medication because I am MORBID OBESE (at least that's what the chart on the back of the door said.) I have been taking phentermine for 2 weeks now, and ive only lost 9 pounds. The weight loss is NOT steady, I will lose 2 pounds and gain 3 back the next day. Im not hungry at all! I am forcing myself to eat so I don't put my body in starvation mode. Before the diet I probably ate more than 3000 calories a day. Now, since ive been on Phentermine,I try to eat about 1000. There has been days where I can only eat 500. What is the least I can eat that my body wont go into starvation mode. My diet has stalled and I don't know if im eating too much or too little. All the Dr. said to me when he wrote the prescription was "Watch your calorie intake but dont eat too less or you body will go into starvation mode."