There have been threads on this here before.
My experiences seem typical:- Disturbed sleep patterns (dozing after a work shift for example), wake up unable to move, sensation of a "presence", or of a weight pressing on me. Extreme fear. It all fades after a couple of minutes and I wake fully and can move normally.
First time was very, very frightening. Subsequent events much less so, because I recognise the phenomenon. (The fact that stroke victims describe similar paralysis is of course worrying as I get older, but stroke does not seem to have the associated sensation of presence or suffocation-the "unknown force" OB describes).
I suspect that force, or presence, is one's self- a normally integrated system which is operating as separate processes in this case. I think the phenomenon may give us a window into such matters as personality, consciousness and schizophrenia.
Jambo's experience sounds quite different to me. Frankly, it reminds me of "the whirlies", which we have all had on lying down after taking too much alcohol- an inner ear / balance problem. Assuming alcohol or drugs are not implicated, I would suggest that if it gets worse or more frequent, a doctor should be consulted.
Any growth spurts in the last year or two, Jambo? Remember teenaged nervous systems are often changing at an incredible rate to cope with body growth. It can have wierd effects. You literally change from day to day. Great fun. Once was fine, thanks.