I'm a travel nurse on an assignment in TN. I've worked over 100 hours in a 10 day time-frame and was told that because the hours were unauthorized,

It depends on the employer's size and what the employer knew or allowed.  Tennessee does not have its own wage and hour act which means federal wage and hour law is the only option.  The FLSA only applies to employers with $500,000 or more in gross receipts.  At that level or above, then you may be able to file a claim under the FLSA. The FLSA, in general, requires an employer to pay an employee for all hours worked if that employee is non-exempt. It is "in general" because there are some exceptions such as when an employee has been ordered not to work or the employee is effectively hiding hours from the employer.  As for exempt employees, simply paying a salary is sufficient.  If your employer has gross revenue higher than the above, I would recommend at least talking to an experienced employment attorney about your situation.