A Witch Rode My Back Last Night

Well that what my grandma use to call it. It's when your sleeping and you can't move but you can hear everything around you. It is very scary and it seems like there nothing you can do. This has been happening to me for several years now but it had stop for at least a year but then it happened last night. I was sleeping and it felt like I was awake I heard my boyfriend moving around me and I kept yelling at him to wake me up. Then finally I relaxed and I broke my self out of it. I did a Google search for "can't move while sleeping" and discovered that I actually had sleep paralysis.
Sleep paralysis is common condition characterized by either partial or total paralysis of muscles. It occurs upon awakening from sleep or falling asleep, although the latter is much less likely. The episodes generally last anywhere from seconds to minutes, and outside stimuli like touch or sound may terminate it. According to Wikipedia, sleep paralysis "it is closely related to the paralysis that occurs as a natural part of REM sleep, which is known as REM atonia. Sleep paralysis occurs when the brain awakes from a REM state, but the body paralysis persists. This leaves the person fully conscious, but unable to move. In addition, the state may be accompanied by terrifying hallucinations (hypnopompic or hypnagogic) and an acute sense of danger."
Symptoms
Sleep paralysis entails one or both of the following:

•Paralysis: occurs after waking up or just before falling asleep. You cannot move any body part, aside from involuntary movements such as blinking and breathing. "This paralysis is the same paralysis that occurs when dreaming. The brain paralyzes the muscles to prevent possible injury during dreams, as some body parts may move during dreaming. If the person wakes up suddenly, the brain may still think that it is dreaming, and sustains the paralysis." (Source: wikipedia.org)

•Hallucinations: Images or sounds that appear during the episode. The sensation that someone is standing beside you or somewhere close by in the room. Strange sounds. Some people feel a heavy weight on their chest, as if they are being sat on.
Causes
Although there are several theories, the actual cause of sleep paralysis is still unknown. However, several factors have been identified that may increase the chances of having an episode:

•Sleeping in a face upwards or supine position
•Irregular sleeping schedules; naps, sleeping in, sleep deprivation
•Increased stress

•Sudden environmental/lifestyle changes
•A lucid dream that immediately precedes the episode

Now that I know what this is, I need to figure out what triggers it. If you have ever had this to happen to you hopefully this information has given you a little more clarity.