It could be from a lack of Vitamin K or Vitamin C, especially if you don't eat green vegetables.
Vitamin C needs to be taken daily, but Vitamin K is stored in the body.
I once stopped the nosebleeds of 2 girls who were having them daily, by buying a bottle of chlorella capsules and having them take six each per day until the bottle was empty. The nosebleeds stopped after the first day, and the rest of the bottle got them well stocked up on Vitamin K. I then advised them to eat green vegetables every day.
Chlorella or other green algae supplement contains a lot of Vitamin K with other nutrients that help it work better. You can also buy plain Vitamin K capsules.
Vitamin C strengthens the blood vessels and capillaries, so they are less likely to be damaged. Vitamin K is essential in getting the blood to clot, so if bleeding starts at all, it will probably stop before you notice it.