Why am I gaining weight?

I am very confused right now. For about 3 years, my weight had been steady at between 106-110 lbs. (I am 5'4). I had played soccer for 18 years, so I have always been pretty athletic. I stopped playing 2 years ago, and had been pretty inactive until last summer - I started using an elliptical machine for about a half hour 2-3 days a week. Everything was fine until I started to gradually gain weight, reaching about 113 lbs. by January and 117 by March. Six weeks ago I intensified my workouts, and am now working out pretty intensely. I strength train (full body, free weights, low weights with 12 reps and 3 sets) 2-3 days a week, play sand volleyball with friends 1-2 times a week, and run about 10-13 miles a week: I jog 2 miles on strength training days, do interval training 2 days a week (for 30 minutes: Jog 2 min., sprint 30 sec.) and take a long run (4-6 miles) one day a week. I am gradually gaining weight, and I don't know why! My clothes are tighter, I have gone up a pant size and a shirt size. I tracked my food intake with a food diary for about a week, and on average I take in about 1100-1200 calories a day. I eat very healthily: typical breakfast is either instant oatmeal with fruit or a homemade smoothie made with 1 cup fruit, 1 cup light soy milk, ice, and a packet of No Sugar Carnation instant breakfast. Lunch is a salad with protein, I eat a snack when I get home (before working out), which is usually a high fiber or high protein breakfast bar at about 150 calories. Dinneris typically grilled chicken or pork with steamed veggies and some type of whole grain, or steel cut oatmeal with fruit and nonfat yogurt. Am I working out too much or not eating enough? I'm scared to add more food because I fear gaining even MORE weight!

Why am I gaining weight?
Educating yourself about how carbs vs protein effects your body is the first place to start. If you did have a real dietitian come up with a meal plan for each day, how much would that cost? Hundreds of dollars. The internet has made it so easy to find information these days I finally came upon a site with relative tips.