Do I need to eat more or less to lose weight?

I've been slowly gaining weight no matter how much I workout! Do I need to workout more or eat more? My workout have included 1hour aerobic class 1x a week, 1hour weight lifting class 2-3x a week, and running on treadmill 30 min. 5x a week. I have gained about 5lbs and can't seem to lose it. I am a 23yr. old female 5'7 and weigh 131lb.  I eat 3 meals and 2 snacks a day with an intake of about 1500-1700 calories a day. I know some of it is muscle but I've gained some fat as well. Could the extra pounds be due to not eating enough? I am welling to lose muscle to lose weight. HELP!!

 My meals include: Breakfast: oatmeal and 3 egg whites, Snack: (south beach high protein bar) Lunch: salad with 3oz. tuna, high fiber crackers, and yogurt. Snack: Banana and Dinner: chicken or fish with green veg. and sweet potatoe I also usually have 1 tbs. peanut butter sometime throughout the day.