If you had Robitussin DM Max, which is what it looks like, you ingested a lot of Guafenesin as well as the DXM. Being as Guafenesin is an expectorant, take too much and you'll get nauseous, get too nauseous and you'll get sick. Next time please pay attention to the active ingredients and don't go chugging down syrup without knowing what's in it! There's a few good syrups out there, though I'll never be able to drink em. Nasty stuff IMO.
Anyways, seeing as I'm a few days late, I really hope you're okay.