Did a doctor diagnose you with serotonin syndrome? I'm dubious, because it's a pretty severe and rare condition. More likely you might have been affected by the alcohol in it. So again, were you diagnosed as having serotonin syndrome? And sex and exercise have nothing to do with this, serotonin syndrome usually occurs when someone mixes different drugs that target serotonin receptors, which to my knowledge Robitussin does not.