There's an old rule of thumb that a sedentary woman needs 8 cals per pound of body weight per day to maintain her weight, and a moderately active woman needs 10 cals per pound per day. So by this rule and knowing that you exercise 3 times a week, you might require 1350 cals/day to maintain your 135 pounds.
If you eat 1200 cals/day and you require 1350 cals/day, over the course of a week you would have a deficit of 1050 cals which translates to only a 1/3 of a pound loss (a pound of fat being 3500 calories).
If you eat 600 cals/day, then over the course of a week you would have a 1.5 pound loss.
If you feel uncomfortable at 600, maybe you can eat a little more and exercise more ? That would actually help you gain (or at least retain) your muscle mass which will help you burn up more calories.