You might have gained muscle! Don't focus so much on your weight, but how you look and feel, and how your clothes fit. I too was wondering why I haven't lost more when I worked out sooo much. Turned out I gained muscle in my legs, and I was under eating! So that could be another thing. Even if you're eating healthy, but you don't eat enough, your body will go into starvation mode and store every calorie!
Good luck to you, and your challenge :) I was going to join, but mine started as the same time as yours! And it kicks my butt hardcore lol.