Hi Jessica,
Good job on sticking to your caloric intake and with your exercise regime!
I'm not sure why you are gaining weight, but I can tell you that the scale is not the best measurement of weight loss since it can be impacted by many external factors. Instead, a better judge of how much you are progressing would be to see if you've lost body fat. Have you been taking measurments of your hips, thights, waist and arms on a weekly basis? Are your clothes fitting looser/better? Do you feel more energized, and are you getting stronger/gaining more endurance? If so, you are on the right track.
I do have a couple suggestions, however, to help jump start your program. For one, I would incorporate weight training into your weekly regime (two days of full body workouts). Try the exercises right on this site under "mystrengthtraininglog" - and do three sets oif each exercise with a weight that challenges you for 12-15 reps each set.
Second, try to get your HR in the aerobic zone for part of your aerobic workout, (which is about 75 percent of your max) or do some days of cardio more high intensity but at a higher heart rate. You don't need to work out for 1.5 to 2 hours per day to get great results. And, if you can shake things up a bit, try incorporating hills and some speed work into your walks.
The important thing is to stay motivated and keep up the great effort. You may need to give yourself a while to start seeing some big results, but as long as you stay focused, you will.
Good Luck,
Paul