I upped my calorie intake & gained weight

How much more did you add? I tend to do best between 1500-1800. more than 1800 and I tend to "maintain".

If you work out a lot, it could be water weight in your muscles. I had one week where I was just killing it at the gym almost every day that week, and my weight wasn't going down - it went up 1.5 pounds, actually. Then, when I took 2 days in a row off, I dropped FOUR pounds. It has to do with the inflammation and swelling in your muscles after a lot of hard workouts without a rest day. They hold water like a sponge at that point, which means that it hides any fat you've been losing. Also, if you have a monthly cycle (as in, you're not on a birth control that prevents it), you can gain as much as 5 lbs on average during that week of the month. It falls off quickly, but it can be really shocking to see on the scale if you're not expecting it.
Also, if you've been consistently losing weight for 3-4 months, your body may need a month off. About every 3 months I go to maintenance for 1-2 weeks to let my body relax. As soon as I cut my calories again, I get those impressive 1-2 pound per week losses.

Also, try cutting your carbs to less than 100 grams per day for a week. It's hard to maintain that for more than a week at a time, but whenever I do that, I lose about 2 lbs. Eat a lot of high-fiber and high-protein meals, with very few carbs, and only veggies and fruits for carbs. Avoid oats, breads, etc. After a week of that, it tends to kick start your weight loss.

People mistakenly give this advice of eating up because they thought they lost weight when they ate more, but it was really just normal fluctuations. The bottom line is that if you have extra fat to lose and the TREND is not down on the scale then you need to eat less. Ignore exercise calories because they are over estimated. Most people underestimate calories so give it 10% less in the total. Also packaging will underestimate because legally they can put more not less so they will error on the more side. I learned this all from a book I describe below, written by someone with multiple PHD's and knows his stuff and I've had a chance to talk with the author.

Your body loses weight in chunks, not linear. I have found that you can do everything right and your weight loss seems to plateau but you are patient and keep exercising and eating at a deficit (however slight) you will lose it, it will suddenly "whoosh". There are so many variables for the scale though: water retention, digestion, allergies, sodium, carbs, water intake, DOMS, inflammation, the list goes on. People mistakenly think they lose weight when they eat more because of these fluctuations.

If you have a lot of body fat reserves you would be surprised at how little you can eat (unless you have emotional eating issues or disorders). The leaner you get the less your body has to draw from and then you have to taper up your calories. There is no such thing as starvation mode for women over 12% body fat or men over 6% body fat. I am a living breathing example of that. I went from obese to now under 12% BF and I've maintained for one year and built muscle the whole time. You don't BUILD muscle in starvation mode, so I proved everyone wrong.

So did all these ladies ---> http://www.venusindex.com/transformation-contest/
It costs nothing to read and listen to their interviews and it’s fun because they are real like you and I.

If you want to learn the real truth invest in this. I gain nothing by it, but I am passionate about other people finding the same success I did and learning all the myths floating about in the diet industry that is designed to keep getting your money over and over.
--- > http://www.anythinggoesdiet.com/no-diet-rules-allowed.php
(This is NOT about eating junk food! It's about learning to eat for you own lifestyle, vegan, organic, whatever)

My story here --> http://www.venusindex.com/roberta-saums-venus-index-transformation/

Oh yeah and you can do this WITHOUT a personal trainer or coaching. Read the book, it explains the answers, done. Then just do it. Of course the superior designed workout helps as well.

Maybe you aren't tracking your calories correctly?

how long did it take to gain 3kg back?

This.

When you change a variable, you need to give your body time to adapt to it. All of the people I know who have increased their calorie limit saw an initial gain, but eventually the weight came back off. Your body will often respond initially to change by gaining weight. Give it time, 4-5 weeks or so, before you say "this isn't working".

That happened to me,too! I just did a different exercise video and that helped. Who really knows,we are all so different,guess that's why we have this weight situation!

So you're saying if I keep doing what I'm doing, the weight I recently gained will just come off because it's still adapting? Just making sure.

what are your stats (age, weight, height, bf% if you know)? Who decided you weren't eating enough and why?

So you're saying if I keep doing what I'm doing, the weight I recently gained will just come off because it's still adapting? Just making sure.

Sort of. But you also need to make sure that the change you made makes sense. In your case, 1200 calories is probably low. But I don't know what you increased your calorie allocation to. Also, burning 200 calories a day is not such a great deal of exercise that you need to be concerned about fueling your workouts.

If you were losing weight before, why did you change your calorie intake? Don't change what works, unless you're miserable.
How many calories are you consuming now? And are you being VERY consistent with your calorie intake?