I agree that you are probably restricting your calories too much, so may be losing water or muscle, not fat. We are all different, so you need to figure out how many calories is adequate for your own body. There is no set amount of calories that you should stay above or below -- I'm very small boned and my doctors have always told me not to go below 1000 calories/day; however, someone who is taller/bigger than myself, may need to stay above 1500 calories/day. If you don't know how to calculate your calorie needs, let me know; I'll be glad to help, or you can do a "google search" for calorie calculators.
You are losing weight. So you definitely lost inches. I think you are measuring your waist and can’t find difference. I think you have to be careful more about your exercise in gym. You should follow the exercise which is specifically for belly or waist. And please consult a doctor to confirm that you are not loosing your muscle.
The first and most obvious reason may be that you're measuring wrong. It's easy to over or underestimate your measurements. Plus, depending on the size of your frame, 7 lbs may be hard to measure.
Also, you could be losing it in places you aren't measuring, your face, bust, neck, etc. Fat cells shrink wherever the heck they feel like shrinking.
How many calories are you eating per day? This is my usual lecture. :) If you're restricting too much, you may be dropping pounds in water/muscle but not as much fat. We don't want that. You should not net below 1200 per day.
Otherwise, just give it time. You've probably lost a quarter of an inch here or there, and you're just not noticing it.