Most often, when the number on the scale changes, it's due to fluctuations in the amount of water, glycogen (stored carbohydrate), and waste in your body, which shift from hour to hour and day to day. Losing actual body fat takes longer, because the only way to burn excess body fat is to create a calorie deficit, and that can be much trickier than it sounds.
Consuming too many starchy foods, such as potatoes, rice, pasta, and breads (especially at one sitting), provides your body with more than it needs for energy and glycogen stores; anything left over will be stored as fat. "You don't have to eliminate starchy carbs completely," says IFBB pro Mike Matarazzo. "But you should really cut back on them when trying to shed body fat." Limit total starch servings per day to 3-5, where a serving size is one cup of pasta, rice, or sliced potatoes.
What does a HIIT workout look like? You could jog for two minutes, sprint for one minute, jog for two minutes, sprint for one minute. Or you could do a HIIT workout on a bike, or by running up stairs and then jogging back down. The key is that you go relatively all out for a short period of time, then recover by maintaining a moderate level of intensity, then go again.