It is possible it is your thyroid. If your doc is testing your thyroid be sure they do a T4 free, T3 free, TSH and thyroid antibodies. Most the time the tests that docs do don't include the "free" tests, these are the tests that measure the active thyroid hormone in your blood, so you want these done. Also, you can have "normal" numbers in the other tests but still have a thyroid problem if you have antibodies, so you will want them to check for antibodies.
Good luck to you.