It could simply be re-calling bad memories. To making your own. Red dreams are long and tourturesome, and over-all very creative.
Your Red Dream could be about school or work. The classic "I forgot my ________ (clothing item) when I went to school/work" is a common one.
Red Dreams (nightmares) can be about someone (or something) dying, supernatural beings so weird, strange, and real that they are terrifying. Either way (any way) Red Dreams are NOT a pleasant experience.

Red Dreams can be caused by a frightening book, story, or happening, a bad encounter, and embarrasing moment, bad carma, or anything, really.
Sometimes red dreams are not bad (at points). But then it could be social exclusion and/or physical pain later.
Red dreams are caused by either a very creative mental spur or a recent, sudden tradgedy. Also from simply watching a scary movie or reading a scary story. As you see, the cause of a Red Dream can vary widly.

However, you can always tell a Red Dream/Nightmare from other dreams for their long and tedious personalities. They also leave you emotionally weak in the morning.