while this may be frightening and unexpected a random nosebleed is really nothing to worry about. during sex the blood pressure tends to rise which can be a factor. low room humidity a cold or allergy taking a daily aspirin or minor nasal mucosa trauma can also be factors. if you are having an signs of bruising on your body for no particular reason one might consider a blood clotting issue that will require your doctor to examine you. if you nose is fine and you are not having any subsequent issues then i would not be too alarmed.
56 male nosebleed after sex never had a nose bleed in my life under no medication