What is happening to me? I'm a little confused? muscle or fat?

I am a 16 year old girl 180cm and 59kg. I tried to loose weight last summer. It was dramatic and I was underweight 50kg and I had bones sticking out of everywhere. Now I got a healthier diet and I even workout more. I run 1-2km everyday, I do pilates and crunches everyday. I eat only natural food.. no processed food. But I am a milk addict, I drink like 5 cups a day..(I cant live without milk..dunno why).. I also eat sweetend yoghurt occasionaly as far thats the only junk food I hav eaten, cuz it has sugar. I also go to the gym 3 times a week, I do 20 minutes on total-body-trimmer machine and burn 250 calories, then I do aerobics, and work my calves, I do quad extensions, and on my arms, then I do stretching. and finish. I eat between 1500-1800 calories a day. And 3 months ago my jeans were not so tight and I weighed only 55kg, now my jeans are very tight, and my briefs too. Does that mean my excersise is too high and I'm becoming muscular and bulky or am I doing too little excersise and becoming fat? and how do I loose that weight?