You may be actually eating more when you excercise, in your mind you may be thinking that you can eat more because you exercised. Keep a food log and be sure of what you are eating
When you are exercising, you are gaining muscle, so that may make it seem like you aren't losing weight; however, you should notice that you are becoming more toned and are most likely losing inches instead of pounds. Try measuring you chest, waist, hips, upper arms, thighs, etc and keep track of progress that way, as well as with the scale.