What Causes Pain And Bleeding From A Nipple?

Hello, When I steeped out of the shower a little while ago, my nipples were both hard and they hurt (this has happened before, but only lasts for a few seconds) so I didn t think anything about. But when I began to dry my chest with the towel I noticed one of my nipples was bleeding. It bled a tiny bit for a few minutes after that and then stopped. Could you tell me what this means or if anything is wrong? Thank you.