Am I gaining weight, or just in my head?

K so I've been going to the gym for a couple of months. When I started they found out I was 20 pounds underweight and had 6% body fat. I have abs and muscle, but I was underweight because I barely had muscle. I've been looking at myself in the mirror and notice I have visible and strong looking abs and muscle, but I feel as if I've gained fat. I don't know if it's the muscle that's making my upper body appear bigger, or fat. I work out around 2-3 times a week and swim once a week. I walk a lot too but have gotten an addiction to chocolate. I drink 2 liters of water a day, plus 500 ml of orange juice. I eat 4 times a day. I eat pizza and fat foods on some occasions. That's not what's worrying me. Chocolate is fattening and I eat 1 sometimes 2 bars a day. Bars being like a Recess or Wonder Bar. I used to be fat, so I'm very self-conscious about my weight. Is it just in my head? Am I actually gaining weight?