Since your metabolic rate goes way down when you are sleeping you breathe a lot less frequently. Your girlfriend, who is conscious when she is watching you, is likely comparing your breathing rate with her own, and thus notices the differences.
Sleep Apnea is generally pretty common, it can be fatal, but don't fret until you talk to your doctor.
The University of Waterloo is conducting extensive research on Sleep Paralysis, I have attended some seminars as I frequently experience this phenomena. I often have complete, full-on sleep paralysis where my dreams are superimposed on reality (I hope I am not stigmatizing myself), apparently this is what a lot of ghost sightings are attributed to. This usually happens when I am sleeping on my back; I will slowly open my eyes, I am certainly conscious but not 100%, I am aware of my surroundings but cannot move. I usually feel a crushing weight on my chest and I usually see/feel the presence of another person (sinister) beside me. Always freaks me out so this is how I know that I am not 100% conscious or else I could just lay there and enjoy the show, but I always feel threatened. Up until a few years ago I had no idea what this was and didn't talk about it, but now I realize it's just a minor anomaly, and I don't think I'm crazy.....