Listen to the ISTP.
I have a very 'hale and hearty' constitution so it perturbed me when I started getting nose bleeds in late highschool/college.
It turned out it was a sinus/tooth issue and it worked it's way out.
The first dentist I asked (I hate dentists, btw and listen to my own body) told me it was because of the cold air -- though I had never heard of that before nor experienced it.
So it could really be for a variety of reasons - go to a doctor and leave no stone unturned! I find MDs can have a very laissez-faire attitude when it comes to seeming 'common' occurrences like nosebleeds, stomache aches, etc. even though they aren't (or shouldn't be) common, for you, the patient.