I remember my afternoon Nutrition 101 class Sophomore year of college crammed in one of those stadium seating classrooms with pre-med students, nursing students, physical therapy, and exercise phys people. I think most people knew what a Calorie was after General Chemistry I from the previous year and we had it ingrained in our brains that weight loss was just cutting a magical number through food or exercise. After doing that you will look like Angelina Jolie.
It's not that easy. I struggled to keep my weight within 5 lbs. of what I was in high school with an hour at the gym and probably 2-3 hours of walking around campus (dorm to library and back). Granted binge eating is probably what made it hard but that's another story.
Weight loss really looks more like this. The old 3,500 Calorie rule is a nice starting place but losing weight doesn't take into account water weight, fat, and muscle losses. Metabolism also needs to be taken into consideration.
So you've lost a lb. in one week. What can you expect? Depending on how you lost it (muscle goes first!) will depend on if you gain it back. If you eat ample protein, you've saved that immediate loss of muscle and have killed off some fat. If you had a wicked case of the flu, you've burned off some muscle and have slowed your metabolism a little.
If you're significantly overweight, you shouldn't have issues with the first 10, 20, or maybe even 50 lbs. At some point you'll hit the dreaded weight loss plateau which is your body's way of preventing starvation. This was useful when we were hunting mastodons but not when we're at a desk all day. You'll basically want to eat 100-200 Calories MORE per day and see if the scale changes...or change you exercise routine...or seriously, be honest about your portions and how much you're really eating. It might be time for the food journal.