My only guess as to what happened is that he either was literally applying suction to the labia and for such a long period or so repeatedly that there was a build up of fluid edema; or he bit it, bumped it with his chin or hand or caused some abrasion injury; resulting in inflammation. For it to happen so suddenly and only after oral sex I can't imagine it being anything other than minor swelling, it should go away in a few hours.
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