It's not the pain. Bee venom is what is called a contradictory poison. It inhibits the nervous system while it speeds up the heart and adrenal glands. It has dozens of enzymes in it, including one that turns your cell walls to lecithin and one that causes your interstitial fluid to thin, spreading the venom more effectively. On top of all that, its main component is histamine, which is literally the substance that induces anaphylactic shock.
If you read my response correctly, did I not say venom was the culprit?!

Incidentally, extreme agony can, as well be a contributing factor! For example, extreme pain causes "neurogenic shock" which is an overreaction of the parasympathetic nervous system. This in turn causes bradycardia, etc. Don't talk medicine to me, until you study it...