Why have I started gaining weight?

I have been trying to lose about 10 pounds to reach my goal but for some reason I have been gaining weight. I have recently turned vegetarian because I simply can't stand cruelty to animals. However I eat healthy, I eat mainly veggies, fruits and at times pasta and will occasionally munch some junk food, but not on the regular basis. I go to the gym about 5-6 times a week. I am have started lifting weights but nothing big, about 5 pound weights, for about 2-3 weeks. But is it possible that I could be gaining muscle if not does anyone have a food plan which helps in losing weight or is it actually muscle? Thank you