Terribly Confused / Weight Gain After Months of Exercise and Healthy Eating?

At the beginning of my senior năm my weight fluctuated anywhere from 117-120 lbs. I joined an aerobics class where I worked hard to tone up, and gave up all types of sugary drinks except on rare occasions as my New Year's resolution. However, weighing myself these last several weeks, I noticed my weight has increased to 134 lbs. This has not budged at all. Could someone please explain to me what's happened/happening?