I am working in a salary position, after two months in Family leave due a surgery I went back to work today and I was informed that from Now on I will be working in a lower paying position (hourly). it's that legal?
If your time off was subject to the Family and Medical Leave Act, then no, you should not have been demoted. However, if your employer had a legitimate business reason to change your job to a lower paying position, and it would have happened whether you went on leave or not, then the demotion is allowed.
You can consult with an employment attorney or contact the Arizona office of the U.S. Department of Labor Wage and Hour Division at (602) 514-7100 to find out if you might have a case against your employer.