Muscle relaxant/stress release bomb

I've been curious about this for some time now.  I went through a pretty bad stage of depression that lasted somewhere near two years.  I'm still recovering from it and have to be very cautious about my well being.  I wasn't sleeping at the time at the time and I don't recall eating much.  In ernest I can hardly remember anything from this period.  However, something thats stuck with me; whilst at my worst I experienced something rather unusual, when I was, only what I can describe as; feeling the most awful I could ever imagine feeling and just when I felt it was too much to bear, a particularly strange thing would happen.  There seemed to be a release of some chemical or another and I'd instantly relax and feel better (essentially an orgasm originating from my brain).  The effects only lasted a short time but I'm sure this stopped me having a heart attack or something.  Does anyone know what happened here.  I'm a biomed (That is NOT a Dr) and am looking for some insight into what this process/molecule is?  Any ideas?  Can't find anything online about it.