Almost everyone has experienced something traumatic in their lives, although not everyone reacts the same way--that's why when two people go through the same trauma, one may develop PTSD and the other may not. Also, PTSD can have a delayed onset, so symptoms can develop months or even years after a traumatic event, which can make it seem like it came out of nowhere.

That being said, PTSD symptoms have a lot of overlap with symptoms of other disorders. What particular symptoms are you having that make you think it might be PTSD rather than something else?