Stuck at the same weight

My weight has been fluctuating a lot lately and I noticed I'm getting bigger and eating less healthy foods. For the past year and a half, I've been stuck at 145lbs. even after dieting and some light exercise to get to 135 which is the weight I felt really comfortable and healthy at, I wouldn't lose anything. Nowadays, I'm 145 one day and 150 the next. Why does it do that and why can't I get back below 145?