What Could Be The Blood Blister On Nipple?

Male 39 yrs, I had a blood blister on my left nipple for two days. I first noticed it in the shower after a very light morning exercise. For preventive purposes, I taped a magnet on the nipple last night and this morning there was dry blood on the tape and it seemed that the blister bumpiness went down. I decided to do light exercise again, the blister became engorged again. Should I be worry about this?