Orthostatic hypotension isn't really relevant when a person is lying down. This could be a cardiac issue. Please consult your cardiologist. You most likely need to have a holter monitor test to rule out cardiac causes of pre-syncope and syncope, and possibly a stress echocardiogram as well.
In rare cases, people will feel faint even whilst lying down with autonomic dysfunction, but it's necessary to rule out cardiac issues before assuming this is an autonomic issue.
Hello,
Have a quick look at
http://en.wikipedia.org/wiki/Orthostatic_hypotension
This may help you understand your symptoms.
I hope after reading this you will better understand such symptoms. If you have continuing concerns please consult your Doctor.