Sense memories are more powerful than most realize. They can be triggered by both the conscious or unconscious mind - thus in a dream state. Interestingly enough, it is not uncommon during open brain surgeries (in which the patient is kept awake) for these kind of experiences to occur with sometimes startling results (ie, a patient retching after experiencing the distinct aroma of vomit after the surgeon tapped the section of the brain that apparently catalogs things like that).
As for PTSD, it is characterized by specific symptoms in the wake of an extremely traumatic event in which the individual witnessed an event that caused serious injury, death or threat of death, or had the physical integrity of themselves threatened in similar ways. Initial response is generally intense fear, horror and/or helplessness, avoidance of triggering stimuli, persistent reexperience of the event, and specific symptomotology that is persistent for no less than 1 months time. The next version of the DSM might include a new differential diagnosis for a chronic form of PTSD as well, allowing for the experience of various symptoms over the span of years, which (in my professional opinion) is actually a far more accurate descriptor as I tend to see people seek professional help years after the traumatic event, not within weeks or months. In point of fact, I do not believe I have ever had a woman come in for rape counseling any sooner than 11 months after the assault.
I do tend to wax rhapsodic on this topic, so I will end this here, but if you want more info on the symptomotology, I will be more than happy to provide it!