Suggest Treatment For Low Thyroid Levels

Thyroid ablation 15 plus yrs ago. Recently thyroid level was very low..Dr increased my synthroid from 75 to 100 MCG. Thyroid level normal 2 mos ago but lately having occasional burning in upper chest and occasional feeling like I have slight difficulty w getting deep breath..could I be taking too much thyroid? I do have "silent" heart murmur. Blood pressure normal w bp RX. Thnx. YYYY@YYYY