We don't really handle just DXM questions here, they belong in psychedelic drugs, but as this is more of a serotonin syndrome question I will answer it.
Serotonin syndrome is at the severe end of a continuum from mild side effects through to a fatal disorder.
All about serotonin syndrome
Main symptoms are high temperature, agitation, fast heart rate, delirium, hallucinations, overactive reflexes, loss of coordination, vomiting/diarrhoea and fluctuating blood pressure.
If you've had mild symptoms of it but not the syndrome itself you might not have noticed. If you had the full blown syndrome, you would have been very unwell indeed and needed hospital care. It can be fatal.
The fact that you are unsure means you won't have had the full blown serotonin syndrome, and you definitely don't need to worry about brain damage or anything like that. Be careful in the future though - just because you got away without developing it in the past, doesn't mean it is safe to carry on mixing DXM with serotinergic drugs..
