You may want to get a medical checkup. It could be something as simple as having 'too much blood' in your body. Or it could be indicative of your heart skipping a beat. But as far as the nightmare itself, it could be as simple as your having inner resolve, having great concentration and faith in overcoming the upsets presented by life.
i looked up some sleep stuff on web md and found this "Many may notice the feeling of falling during this stage of sleep, which may cause a sudden muscle contraction (called hypnic myoclonia)" it is part of the first stage of sleep (source- http://www.webmd.com/sleep-disorders/guide/sleep-101)
Standard case of falling in your dream and waking up with a jerk. Its an involuntary muscle spasm and happens to absolutely everyone.
