If you used PTO (i.e. sick time or vacation) to cover your absences, then you should not have had a problem with your employer. However, if your time out was unexcused, then your employer was within its rights to write you up. While seemingly unfair, it was perfectly permissable. The fact is that doctors' notes are not legally binding on a company. Therefore, unless you have protection under the terms of an employment contract or union agreement, you have no claim. A company can set the conditions of employment much as it sees fit(absent some form of legally actionable discrimination).