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Actually, you get nightmares every night. Every time you go to sleep for at least 30 minutes, you will inevitably start to dream. However you will only be able to remember a dream if you wake up during it, otherwise the brain files the dreams deep into its memory. During our sleep, our brain shifts between two types of sleep, REM sleep and slow-wave sleep. These shifts occur about every 30 minutes until we wake up. Slow-wave sleep is when we have most of our nightmares, specifically in what's known as stage 4 sleep. Stage 4 sleep is usually when children pee their bed or have night terrors.

Furthermore there has been no research finding that covering you chest with your hand makes you more prone to waking up during slow-wave sleep, so there is no evidence that you will have more nightmares that you can remember when you wake up by covering your chest.

I know this is going to sound really weird, but my mom has always told me it is negative energy in the room bending your state of mind to project negative and scary images.

This is one of many theories. Some people believe that certain foods cause nightmares as well. Others think that if you have a negative day you will be more prone to nightmares too.