If you get up very quickly from sleeping, you should really try to sit on the edge of the bed for a minute or so to regulate your low sleeping blood pressure. It could be that just the abrupt change in positions, lying down to standing was enough to cause the faint. You have to get your heart revved up enough to pump your blood so can stand up stand up.
I have also heard about peeing causing fainting, but the more logical cause would be just a matter of doing things too quickly.
I second what Nancy said, and will add some personal experience. About the time that I learned I would have to have surgery, I got out of bed to dress for work one morning, and immediately felt wobbly and laid down. Then I felt like I was so dizzy I could not get up. My wife called the ambulance and they took me to the hospital and checked me out. Nothing was abnormal. Heart function was fine. ER doc said maybe it was flare-up of inner-ear imbalance (which I had had over the years).
In looking back, I think I did jump out of bed too quickly, and it may been a low blood pressure issue at first, followed by a panic attack. After going through this and realizing stress might be throwing me out of kilter, I calmed down and had no repeat of an experience like this.
Obviously you should take seriously what happened to you and get the evaluation of your doctors. But I do think people are at their most vulnerable when getting up in middle of night or early in the morning and going to the bathroom. I could recite some horror stories but I won't. Just do try what Nancy suggested about getting out of bed slowly, and also try some relaxation techniques. Talk to your doctor about your stress levels, which seem to be high. Perhaps he would suggest a med, or non-med alternatives -- relaxation techniques, etc.
I am not pretending to have the answers for you -- just recounting my own experience and offering my thoughts. I wish all the best to you.
Adding this as an edit: Here is a link to an article about many possible causes of fainting, and sure enough there is even a name for the malady of fainting after peeing. Doesn't mean that's what happened to you -- there are many possible causes. By all means, ask your cardio:
http://www.medterms.com/script/main/art.asp?articlekey=7661