By working out, do you mean cardio or weight training as well? If you are lifting weights, you may have put on some muscle, which is a good thing!
Before I screwed up and gained 30 pounds back, I was going to the gym faithfully for both cardio and using the weight machines... I was still large, but felt much stronger and was losing inches faster than pounds.
Also, unfortunately monthly timing can obscure your actual results as well...