I wish I had your problem!
Increase the amounts of nutrient-dense foods in your diet. How about 1/4 cup of almonds eaten along with a string cheese? What about peanut butter as a spread on your multi-grain bagel? Scrambled eggs with lean sausage and mozzarella?
If you're eating a high quantity of lower-cal foods - vegetables, for example - they're probably doing a good job of filling you up from a volume perspective. I hope this helps. It's also possible, like the poster above suggested, that you should add in the calories slowly to get back up to a reasonable number.
Good luck!