Our bodies are designed with systems to help us go into self-preservation mode when threatened. Your body likely has picked up on the fact that it's not getting as many calories as it's used to (and as many as it needs to fully meet all your energy needs) and has thus pulled back on the gas by using calories more efficiently. It all goes back to the cave man days when food supplies were unpredictable - and our survival depended on our bodies being able to adapt to feast (store excess calories as fat to be used later) or famine (don't burn as many calories when food intake is reduced). Your body senses you're in famine mode and acts accordingly.
What to do? It's a natural inclination when hitting a weight loss plateau to decrease calories even further (people start to "diet on the diet") thinking this is what's needed to spur continued weight loss. What we have found in our weight loss program is that if patients actually increase their calorie intake a bit for a period of time, they often can get the scale moving again.
The way we explain it is that your body sometimes needs more calories to keep burning calories and over-ride the body's "starvation mode" response. Try adding some additional lean protein or a serving of whole grains each day (protein and whole grains take longer for the body to digest, helping us to feel full for a longer period of time). For example, you could add some broiled or grilled strips of chicken, or tofu, or a chopped egg to a salad; or have a bowl of oatmeal or other whole grain/high fiber cereal with skim milk as a snack. Try it for a week or so and see if the scale doesn't indeed show you are once again on the way to your goal.