Hi! It sounds frustrating what you're going through. The weight gain can be caused by various things--maybe you're building muscle (which weighs more than fat), maybe you've hit a workout plateau, it could be water, or you're eating more because you are exercising.
If it's only a few pounds, don't panic, as long as your noticing that your muscles are changing. Muscles weigh more, but they also help you burn calories because it's harder for the body to maintain muscle. How is your diet? If you're limiting the refined carbs or sugars and adding more fiber (fruits, vegetables, whole grains, etc.) into your diet, then don't worry if you're hungrier, because it's your body's way of compensating for the exercise.
If the weight gain comes from hitting a plateau, you can try and spruce up your cardio routine by adding intervals. During your walking sessions, have a few 1-minute intervals where you run, jump, or squat as quickly as you can (be careful with your knees during the squats). The point of intervals is to get you out of your comfort zone, and it is effective and you may find that you will begin to lose weight. Also, it's a nice supplement to your 30-day challenge.