Almost certainly because you are hyperventilating. I have had two girlfriends who had this-- fortunately with the second I had a good idea of what was hapenning... your orgasms will likely be even better and more enjoyable (without the head-lightness, orgasms and post-orgasm feelings seem to be more intense) if you can keep the numbness from happening-- by either consciously controlling your breathing, or breathing partially into a pillow/arm/whatever so some of what you're inhaling is recycled. I'd suggest you do that, because thinking about your breathing is a bit of a distraction from pleasure!
Euphoric feeling is always what happens during intercourse when an orgasm is achived. Not sure of a numbing feeling but it sure does send a chill through you.