Can having high metabolism be confused for thyroiditis?

Yes. Thyroiditis cause both slow down or speed up metabolism. A simple blood test (tsh, t3/t4) will determine if thyroid disorder is present.
No. Thyroiditis is a condition which presents with specific thyroid-related abnormalities. Fast metabolism may be associated with an overactive thyroid, but that's easily identified through blood tests and would not be confused for other causes of fast metabolism.