Why Is My Nipple Bleeding?

Recently over the past few weeks, I have noticed that during and after runs, I find that my nipples get hard, sensitive, and that my right one has actually bled a few times.

This is a really embarrassing and unusual experience for me. I mean, when it's cold out I've experienced my nipples getting hard before, but never like this.

I was wondering if this is a symptom of something serious because I have never experienced this before and it's making me worried that there might be something wrong.

Any advice or guidance would be greatly appreciated.

Thank you