Nightmares are dreams which means there your mind explaining thing symbolically while working on things in the background. nightmares stem from stress, fear, and life events that were tragic. Don't trust those online dream dictionaries every bodies symbolism in dreams stem from something completely different than what they say. Trust me i've written many papers on dreams, i even diagnose my friends dreams sometimes, along with that i even helped my friend get into a Lucid dream once.