What Is The Skin Tag On The Nipple That Has Turned Red?

I have had a skin tag on the edge of my nipple for quite a few years and have never thought anything of it. I have just noticed that it has turned black as though it has been bleeding and is also painful to touch. Is this something to worry about?
I am 29 years old, 5ft and weigh 8 stone. Always been healthy and never had any medical problems apart from about 2 months ago when I had a loop biopsy to remove some abnormal cells.