Cytomel and radioactive iodine treatment

I have been taking synthroid and cytomel for years now. Had hyperthyroidism (Grave's disease), had radioactive iodine treatment to leave me without a thyroid. About 6 months ago, I started getting hives. Doctors at first thought it was stress. I am not allergic to anything. The only medicines that I take on a regular basis are synthroid and cytomel. I think it might be because of these drugs but not sure. I have made an appointment to see my internist.

I am a 55year old female. Had total thyroid removed on March 2 because of papillary carcinoma. I take 25mcg of Cytomel, three times a day. I have been on it for a week.
I get very bad muscle aches, and slight headaches mainly at night. They are however, beginning to ease up a bit. I will have a radioactive iodine treatment in about five weeks. I don't know what to expect. Help!