An EEG, or electroencephalogram, looks for seizure activity in the brain by monitoring electrical activity in the brain using electrodes pasted on the surface of the head. A sleep EEG is done while the person is sleeping. Here is more information:
http://www.medhelp.org/medical-information/show/899/EEG
(For those in our forum who aren't familiar with the initialism "PLMS," that's periodic limb movements in sleep, also known as PLMD, or periodic limb movement disorder.) I'm guessing PSN is polysomnography? Is that right?
Several of the types of fainting you describe—nearly passing out from pain, almost passing out from vomiting, and fainting from shock—are forms of vasovagal syncope that are generally considered to be episodic and not representative of an underlying pathology per se. Perfectly healthy people can be brought to the brink of fainting or may actually faint under conditions of extreme bodily or "psychological" stress, such as from being dehydrated, extremely overheated, donating blood, seeing blood or gore, etc. However, some people have cardiac conditions or neurological conditions which predispose them to fainting; these may cause the person to faint under circumstances which would not cause a healthy person to faint, or might lower the "threshhold" for fainting such that it requires much less of the typical stressors on the body to induce fainting (or near fainting).
When you describe fainting from standing too long, how long had you been standing before you fainted? Were there other factors that might have come into play besides just the standing (dehydration, environmental temperature being hot, having recently eaten a large or carbohydrate-rich meal, having recently taken a hot shower, for example)? How many times would you say this has occurred, or if that's too big a number to estimate, how frequently would you say it occurs (say, how many times per month)? Did you ever consult with a neurologist about the fainting, and if so, what did they say?
Have you noticed any changes in your symptoms since you stopped using the CPAP? Was it your doctor that advised you discontinue it, or did you decide on your own that it wasn't helping?