Unfortunately, your employer was within its rights in requesting such a note. The fact is that an employer can set the term and conditions of employment as it sees fit; this includes requiring a doctor's (or dentitst's) note regarding an absence from work. This is known as "at will" employment. So unless this action violated company policy, a union agreement or employment contract, no law has been broken. Additionally, your employer's treatment of you must not have been the result of actionable discrimination.