Get an ultrasound. The blood test does not rule out the presence of thyroid nodules/goiter/inflammation. It just tell you that currently, the throat swelling is not disrupting the ability of the thyroid to produce hormone. The best way to look at the physical aspect of the neck is through thyroid/neck ultrasound. I agree with the need to measure thyroid antibodies to see if you have autoimmune thyroid disease.
Possibilities. Most folks with hashimoto's, especially early, have normal thyroid function tests though antibodies against thyroid proteins are common. People with nodular goiter, which is even more common, usually have both tests normal. Agree that ultrasound is a reasonable next step.
It can still. Be a goiter with normal thyroid tests, and if not, a lymph node or branchial cleft cyst would be possiblities.