Yes, you can sue that coworker for defamation. Defamation is making untrue factual statements or assertions about you--such as the claim that you have a condition or illness which you do not--which statement damages your reputation and/or causes you any monetary losses (e.g. if you lose wages from being suspended or terminated). If you wish to explore this option, consult with a personal injury attorney: the same lawyers who handle car accident and slip-and-fall cases tend to be the ones who handle defamation cases.