Maybe you're eating more than you think you are. When i first starting working out i was eating more, cause I was doing cardio and weight training 6 days a week. and I ended up gaining 5 lbs in one week. I was very frustrated, and didn't know what to do. So i sat down and starting writing everything i ate the previous week. (from what i remembered) and i was eating twice as much. And mostly high calorie-high carb foods. I went on a strict diet the following week. and I dropped the weight in a week as well.
Sometimes we think we eat healthy, but that's not always the case. Write down what you're eating and count the calories, and you'd be amazed how many extra calories you're eating