My guess is that you have an autoimmune disease (like Grave's) that is attacking your thyroid and making you hyperthyroid. However, you need further testing. TSH is a very poor indicator of thyroid status (it's a pituitary hormone). You really need to have free T3 (FT3) and free T4 (FT4) tested. These are the actual thyroid hormones and will give a much clearer picture of what your thyroid is doing.
In addition to those, you probably want to have thyroid antibodies (TPOab, TGab and TRab) tested to confirm/rule out an autoimmune disease.
Since you are not on any meds, and your TSH is so low, it's important to get further testing promptly.
Do you have hyper symptoms?
Have you had FT3 and FT4 levels done and also antibodies tests for Graves and Hashi's.
If so , post the levels and ranges here.
You may be suffering from Thyroiditis (inflammation of the thyroid).
Or if your antibodies are up......Graves Disease or Hashi's.
Post a bit more about yr levels and symptoms and others can help too.