Why is it sometimes when i sleep, all of a sudden jerk out of sleep and my heart is racing like a nightmare?

Hypnic jerks: These sound like involuntary muscle twitches called hypnic jerks, or "sleep starts" -- fairly common. It's unclear why this happens, but 1 theory is that just as you're drifting off to sleep, your brain misperceives the muscle relaxation as a sign that you're falling -- and alerts your arms and legs to stay upright. It's worse with poor sleep hygiene & sleep deprivation. Could see a sleep doctor.

Sleep Apnea: Consider sleep apnea. This is a condition where one's snoring can unknowingly cause upper airway obstruction causing one the need to gasp for air. This is what may be waking you up and your heart is racing after trying to gasp for air. Please see your pcp about getting a sleep study done to fully evaluate your condition.