Katie, it's likely you weren't eating enough! 1,200 calories is the absolute bottom minimum and best suited for very *inactive* people. With all your activity, 1400-1600 may not even be enough.
Have you input your average calorie expenditure into Spark's calorie burn goal? Walking that much might boost your intake even more.
I'm glad you realized that 1200-1300 wasn't enough, so you do understand that eating less won't necessarily make you lose weight. You have to eat to sustain your activity level as well, so make sure you've input all your activity into Spark and then see what your recommended range is based on that.
Also, be patient with your body. We're all making pretty big changes here, and it can take a while - yes weeks - for the body to really catch up with the new lifestyle. I know it's frustrating not to see the results you want, but there are other results like how your clothes fit, your energy levels, your mood, how well you're sleeping, that can be just as good at indicating the changes you're making are having a positive effect on your overall health.
Outside of *maybe* not eating enough, I don't think you're doing anything wrong. I think you just need to take some deep breaths and realize that you are doing great things for yourself and your health, and to give your body time to adjust and begin losing weight at a safe and natural pace.