Memory is not just static snapshots or video on a harddrive, but an active process. As such, it is possible to construct false memories (Brainerd & Reyna, 2005). Some of these false memories can be very traumatic (Loftus, 1996).
There is a lot of research of false memories and PTSD, but most of it is concerned around the accidental implantation of false memories during therapy (for example, see discussion in Hyman et al., 1995), or asking if PTSD-patients are more prone to forming false memories (for example, Jelinek et al., 2009).
Are there any case studies of patients who did not initially have PTSD (and were neuro-typical or 'healthy' in other regards, too) nor experienced any events usually associated with PTSD, but developed PTSD based on a false memory?
Brainerd, C.J., & Reyna, V.F. (2005). The science of false memory. Oxford University Press.
Hyman, I. E., Husband, T. H., & Billings, F. J. (1995). False memories of childhood experiences. Applied Cognitive Psychology, 9(3), 181-197.
Jelinek, L., Hottenrott, B., Randjbar, S., Peters, M. J., & Moritz, S. (2009). Visual false memories in post-traumatic stress disorder (PTSD). Journal of Behavior Therapy and Experimental Psychiatry, 40(2): 374-383.
Loftus, E. (1996). The myth of repressed memory: False memories and allegations of sexual abuse. Macmillan.