Losing Weight?

You could have water or waste retention.  I'd suggest you are overthinking and weighing yourself too often.  Also, the idea of 3500 calories = 1 pound is about right, but it depends on what you eat.  If you eat a lot of carbs, you'll burn less fat. Weight loss and fat loss are not the same things. Your weight can fluctuate a few pounds a day based on the weight of food and liquid compared to what you eliminate. 

I would suggest the following:

1. Keep on your current diet or make small adjustments to decrease carbs.

2. Drink at least 100 ounces of water per day.

3. Don't weigh yourself every day.  Weigh in Monday AM only.  

4. Monitor trends over 4-5 weeks rather than day-to-day

What happens in 7 days really isn't indicative of what will happen during a weight loss program. What is important is what you see over 2-3 months.  Since you're probably about 40 pounds overweight, you should have a goal of 5-8 pounds a month rather than 3 pounds a week. It is better to set realistic goals and be pleasantly surprised rather than set unrealistic goals and feel like a failure.

Per the source calculator, you are eating at a starvation calorie level. Someone your sex, height, weight who works out 5x a week should be consuming around 4000 cals/day for fat loss. What happens when you go more than a few days of under-consuming calories is your body starts using muscle tissue for energy and as you lose muscle, your metabolism slows down (so you burn fewer calories).

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Also, your weight can fluctuate from day to day by up to 5 lbs, due to water weight gains and losses. You also won't see a consistent loss (e.g., 1 lb down every 2nd or 3rd day). You may go a number of days without seeing a change, then a sudden drop of a pound or few. If you're drinking a lot and not sweating it out or consuming a lot of sodium, then you may carry water weight frequently, which will skew your fat loss numbers.

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The faster you lose weight, the harder it is to maintain that loss. Anything over 3 lbs/wk is too fast (and 1 - 2 lbs/wk is better...easier to do in a healthy manner and much more manageable to maintain once you hit your goal weight).

Muscle weighs more than fat.  You should feel a difference in your clothes after about 2 weeks.