@Lindi,
This is quite a common complaimt, regurarily assosiated with something supernatural. But it has nothing to do with it. On my country recives many names "el muerto" "la parca" "la muerte" (death corpse, reaper, death).
I assumed it happened while trying to sleep or while trying to wake up. Is themore common presentation. They are called Hipnagogicc allucinations and Hipnopompic allucinations, respectively.
Although they can happen even in vigil state, and inrelation to enviromental triggers like drugs, stress, fatigue, etc. They can be associated with paralisis, a feeling of coldness, fear, feeling of peace, relaxation, sweating, etc.
We all human work with the same hardware, our brains. Our brains are imperfect, they have bugs. Some of this bugs are common to a great number of human beings. You have nothing to worry about. Antropologically this allucinations, or bugs in our neuronal pathways of the brain had been associated with paranormal activity. Like angels, demons, trolls, goblins, light creatures (some people actually claim to see white figures).
Many ghost, angels,aliens histories are nothing more thanmiss perceptions of our own brain.
Do not be afraid. There is a saying on my country.
"hay que tenerle mas miedo a los vivos que a los muertos"
Be more aware of the living ones than of the death ones.