Have your thyroid FUNCTION tested.
If you only had standard blood tests, they wouldn't
indicate Thyroid function, just serum thyroid hormone levels.
In order to totally rule out hypothyroidism, you need Free T3,Free T4 and Reverse T3 tests.
Insist on these 3 tests, even if your doctor(s) may frown upon your request !
Underlying reasons for this onset could have to do with
certain undiagnosed deficiencies of nutrients/minerals
vital for thyroid function, or excessive stress.
There are other possibilities as well, like kidney issues,
liver issues. cancer, hyperventilation, insomnia, etc.