I had the same problems when using DXM - it would leave me feeling really off for a few days to a week. I'd be depressed and very distant, and just generally feel purposeless and like I was just floating in space. I actually had to really cut down on my DXM use because there were a few months where I was using at least 600 mg at a time a couple times a week, and this left me in a pretty constant state of dissociation punctuated with periods of intense depression. This may be due to underlying psychological issues of mine that were just worsened by the DXM, but I guess it's hard to say.
I'd advise you to avoid the stuff for a while, and see if it goes away on its own. If it doesn't, then I'd consider seeing a psychiatrist to see if meds could help level you out.