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!
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