I have had this before but this afternoon was very different. I never get it during the afternoon when I nap nor does it happen when I sleep on my stomach. But today was a special day to freak me out I guess, It happened in a second, and I heard Heavy Breathing and I felt LIke I was dead, and For some reason I was saying O god. Not fun.

I have heard that 'naps' are better to receive an OBE(sleep paralysis) then an actual night time sleep..
And sleeping on your back, side or tummy does not prevent them..I don't know who ever said that, but I hear 'others' have them laying in all/any position.

No they aren't fun when you aren't 'trying' to have one. Infact it can be most annoying when you really just want to sleep.

I've heard many different things that cause sleep paralysis..from abduction, incubus/succubus attack, and then out of body experience(OBE) personally, I'm hoping it's the latter. Can you remember anything else? Were you really tired? Were you alone? Did you 'leave' your body at all?
Magestica

Actually, a lot (read: most) alien abductions are a result of sleep paralysis. I saw a report on this awhile back. Sleep paralysis is very common, though there are many people who simply never experience it.

It shares the same traits with "Abductions". You know...the feeling of being paralyzed, of being watched, of being moved or feeling like someone wants to do you harm. One woman said she would envision demons pulling her out of bed and taunting her, but her husband slept soundly next to her the whole time.