Increased calorie intake from 1200 to 1500 and....gained?

so, I was on a 1200 kcal diet for about a month and last week increased my intake to about 1500 kcal a day. In the result, though I lost about 12 lbs previously, I`ve gained about 3 lbs back! How come this happened and will I lose weight with 1500 kcal a day? I`m 19, 5`10 and usually work out about 3-4 times a week (cardio).

I have had this happen to me a LOT. I find that it is just a little extra food/water weight. Plus, look at what you are eating to increase your diet. Sometimes it isn't as healthy as we think. NOT saying you aren't eating healthy, but it can't hurt to double check, right?
Also, if you eat a ton late at night, that will make you gain just based off of the weight of the food. 

Do not worry it is just your metabolism adjusting. It will get back down to normal shortly! not to fear :)

Try increasing calories 10% at a time, for two weeks.

So, from 1200, go to 1320 for two weeks, then increase to 1450.

Don't fret, I'm sure all will balance out :)