There are a lot of stories and views about shadow people, and I myself have been "visited" by them in the past. My own view is that they are merely the byproduct of a mind made foggy due to the sleep cycle being disrupted.

When I'm experiencing sleep deprivation I'll often start seeing shadows off in my peripheral vision, or I'll sometimes interpret objects incorrectly, or see face-like patterns in textures (like the folds of clothing). It only happens when my brain is tired though.

There's also something called sleep paralysis. When you sleep, your body enters into a form of paralysis to prevent you from acting out your dreams and unintentionally hurting yourself. However, it doesn't always turn itself off as soon as you wake up. When you experience sleep paralysis, you'll often also experience hypnagogic imagery.

Hypnagogic imagery is like when you're laying in bed and feel like you're falling, so you suddenly jerk awake and kick your legs out. However, when you experience the hypnagogic imagery combined with sleep paralysis, it's usually a very scary experience. You'll see shadow people, or be pulled up into the air (akin to alien abduction), or fall through your mattress and feel like you're about to die. All this is though is your brain being confused as to why you can't move. Since sleep paralysis is supposed to turn off when you wake up, when it doesn't the brain wakes into a situation where your body doesn't respond. This triggers survival mode, and you start hallucinating an explanation for why you can't move.

One of my own experiences with it was that I woke up to a shadow person holding me down against the mattress. I could see it's silhouette next to the bed, and I was positive it was going to kill me right then and there. As soon as the sleep paralysis ended (just a couple seconds) the shadow person was gone and I could move.

It could be that you experienced something similar. Dreaming that you were sexually assaulted by a shadow figure is a common enough occurrence that centuries ago it inspired the legends of supernatural entities like the "Incubus" entity. Thunder Bow could be very correct in that it could be a repressed childhood memory, but it could have also just been a creation of the brain. There are certainly enough TV shows and movies that depict people being attacked that your mind would not lack for the visual reference, even if you yourself had never experienced such a thing.