It would seem that this is not a wise combination to be experimenting with. DXM itself is known to interact heavily with a lot of commonly prescribed medications, so for future reference, do a quick Google before you combine DXM with anything else in the future. Cyclobenzaprine (Flexeril) has been implicated in cases of Serotonin Syndrome for instance, which, with Dextromethorphan in the picture, would probably be your most realistic worry.
I would educate yourself on the signs and symptoms of Serotonin Syndrome so you'll know if you're experiencing a severe complication.