Not all calories have the same effect
I sort of have to disagree with stotalhealth on the calories thing. Having read the exhaustively reserched and very well documented book by Gary Taubes I believe that not all calories have the same impact on health or weight. (edit: book is Good Calories, Bad Calories) Eat more fat, fewer carbs (much fewer grains and other starches especially, and quit sugar altogether), watch your protein quantities. This might shake you up a bit and you may see some slow movement downwards.
And I agree 100% that you are not taking in enough total calories. Your body may be slowing down weight loss to hold on to its own lean mass. More fat and meat, fewer carbs - why not give it a try?