you were not having an overdose, DXM is known to cause nausea and vomiting. You should do some more research prior to putting new drugs in to your body in the future. Good recreational doses for beginners are around 350-450mgs
^ This. Most dissociatives, e.g. DXM, Ketamine and PCP, can all cause a lot of nausea at higher doses, with DXM it seems to be the worst.
You did not overdose, however I agree that you really need to do a bit more research before taking something in future, if you did not realise that nausea and throwing up was common with high dose DXM and thought 800mg was anywhere near an overdose - you can't have read up much about it, which is worrying, as if this was any other drug you could be in serious trouble right now.
Also, the bigger the dose, the longer the "afterglow" - which is pleasant for some, not for others, but it generally feels like a mild high, hence why you think you're still high over 12 hours later. Another thing is, often if you fall asleep on a drug, the effects seem to last a while longer - often remaining until a little while after you wake up, I'm not sure what the science behind this is, and this definitely doesn't apply to all drugs, but I myself and a fair few others have noticed it with dissociatives and opiates in particular. (particularly those with a 4-6+ hour duration in the first place)