hahahhahahah
I've gotten this feeling (though not as extreme as "cumming").
Its when you've lifted weights for a certain duration of time and you muscles become exhausted. You then push beyond that point and when you've done that final set, you literally have a feeling of being "high". I would think its the same feeling runners get when they push themselves in a marathon. Probably the body releasing pheromones due to you pushing your body beyond its limit.