Why am I gaining weight after going on a diet?!

You either gained muscle, OR sometimes, when you cut calories too much, you'll actually gain weight because your body will go into 'starving mode' in which it will try to hold on to calories if it thinks you're losing too much at one time. So try going light on yourself at first and gradually decrease the amount of bad things you're eating. Since September, I've lost almost 20 pounds by gradually decreasing my calories, drinking lots of water (and a cup of green tea a day), walking my dog for about 20 minutes every night, and hula hooping for 30 minutes when I get home from school (but I didn't start the hula hooping until a month ago but BOY does it work!).

So, try going lighter on yourself. I'm 15 and a girl and I've lost almost 20 pounds.

You need to eat more than that, for a start. And make sure you get your protein and vitamin supplements.

Chances are your metabolic rate is so low that your body is programmed to store any extra fat it gets.

Increase your exercise by five minutes every three days for the next while until you're doing 2 miles a day, and then try doing it a little faster.

Right now you should be able to do that mile in 20 minutes or less.

if you don;t eat enough your body will go into starvation mode to conserve energy
when this happens it takes 3 months for the energy to be used and only then will you drop the weight
just eat a bit more but exercise more to burn it off

Maybe your clothes are too heavy that it affects your weight or you are too much diet made you overeat the next meal/day

Waterweight, muscle gain, natural fluxuation

7Watch your portion sizes. Eating a whole bag of rice cakes in one sitting won't help you lose weight. When eating chips, nuts, or dried fruit, put a portion in a small bowl and then put the bag away. That way you won't mindlessly eat a larger portion than you had intended. Even if you only make minimal changes to your diet choices, reducing portion size will inevitably reduce caloric intake. A great way to watch portion sizes while snacking is to buy one serving 100 calorie packages - and they come in many favorite snack food items! Eat slowly to avoid overeating; you will get satisfied with less calories.8Graze on healthy snacks. Just because you're getting healthy doesn't mean you can't snack. In fact, eating small meals and snacks throughout the day, or grazing, has been shown to aid weight loss, compared to eating three large meals a day, by keeping metabolism steadier. Pick snacks that are low in calories and fat and high in fiber dried apricots, nuts, rice cakes, fruits, baby carrots, cherry tomatoes, and so on. Vegetables are generally very low in calories, very high in fiber, and full of flavor and nutrition. Avoid starchy vegetables like potatoes, and try to eat vegetables plain, without fatty dressings or dips. Fruit also makes a good snack. Fruit contains more soluble fiber than vegetables, which slows your body's absorption of carbohydrates, thus releasing energy more slowly preventing sugar highs, and keeping you feeling full longer. Fruit juices are not a replacement for the real thing. You need that fiber, and juices often have more calories than the equivalent serving of fruit and as many calories as soda! Be careful with dried fruits, because without the water, you tend to eat more, and fruits, when dried, are calorie-rich per ounce. With any dry or dried food, be sure to drink plenty of water.9Get more fiber. There are many myths about fiber, but there is science to back up its helpful role in the diet. Fiber keeps the right amount of water in your intestines, making your digestive system work more efficiently and helping to keep you regular. Thus, just eating enough fiber may help you feel slimmer in just a day or so. There is also evidence that fiber in the diet can help prevent stroke and heart disease, ease the effects of diabetes, and may even directly help in weight loss.10Drink plenty of water. An adequate amount of water is essential for health, and a great many people simply don't get enough. What's more, if you're chronically dehydrated, your body will retain water in unflattering places, so if you make sure to get plenty of fluids you can start visibly trimming down in as little as a day. Remember, the more you exercise, the more water you'll need. See the related wikiHow for more details on how much water you should be getting. Drink one glass of water before your meal; it will help digestion and also to get the sensation of fullness in your stomach that will keep you from overeating; that in combination with eating slowly, will allow you to stop eating when you are satisfied, not when you already ate too much (If you eat too fast you can get full, even bloated but still be hungry) .11Write down your targets on sticky note pads and then stick them in most spaces of your home. These stuck notes will always remind you of your goal and will further inspire you to stay on your regime. They would not let you forget your goals at any time.12Exercise. Remember, you can lose weight either by decreasing your calorie intake or increasing the number of calories you burn. Any health strategy should include both, but if you want to lose weight fast, exercise is essential. By making time for exercise and other healthy, productive activities, you can also avoid temptation to overeat.
Perform low impact aerobic activities. Moderate aerobic workouts incorporating brisk walking, cycling, aerobic machines, or swimming not only burn calories they also keep your heart healthy. Swimming is great, especially if you are quite overweight or have joint problems, because you can get the same benefits of running typically burning even more calories with much less stress on your joints. Try to get at least 30-40 minutes of aerobic exercise at least three or four days per week.
Pump some iron.