I am really starting to get frustrated

I had a complete thyroidectomy December 4th 2006. At that time I was a size 26/28. After the surgery and getting over the fact that I had cancer, I was ready to really take charge of my life. I began to do a lot of research and I focused on leading a healthy lifestyle. I adopted a vegetarian lifestyle and I became an aerobics instructor. Within 2 years I was down to a size 14/16 and about 80 lbs lighter. Now, after having a baby, gaining 70 lbs ans losing 70lbs, I have started to gain weight again and I'm not sure what is going on. I teach 4-10 high impact aeorbics classes per week. While I eat poultry and seafood now, I still monitor my food intake very closely. But It's like I am gaining 5-10lbs per month. I recently went to the doctor and he said my levels were normal. I take 125mcg. I just got off the phone with him begging to do something. I really don't understand why I'm gaining weight. He was reluctant but he has decided to let me experiment with 150mcg. Hopefully I will see positive results. 

I've always been told that they need to keep my levels surpressed to prevent the cancer from returning. Does anyone know this to be true?