cant tell you whether you had an allergic reaction for sure... hydrocortisone can be used to treat vaginal irritation and can be used internally in other preps such as preparation H for relieving hemorrhoid pain, so it is used in that area and used internally in certain cases ... but there might be something different in the topical cream that could have irritated you internally, just as a topical skin lotion can cause irritation internally if used as a lubricant.
now, tissues around the outer third of the vagina and around the vulva do swell naturally during sex. blood engorges tissues, much like the mans tissue is filled, and it produces a natural "cuff" that tightens around the man, giving friction. so... im not sure if you were more swollen than normal, whether it was noticed because of skin irritation, or whether you were irritated and more swollen...
anyway, perhaps you have your answer. a topical ointment shouldnt be introduced internally, and it makes for a logical explanation. until it happens again, that seems to be a reasonable answer.