What Causes White Cyst Like On The Tip Of Nipple?

I have a hard, white cyst like ball on the tip of my nipple. Its painless and un-poppable. I have had it over a year now, and it never goes away. It seems to look like it has gotten bigger. On my other nipple however, I get VERY painful ones on the side just before my areola. When I pop it a lot of puss and some blood comes out, then it goes away for a weeks before returning. Should I be worried? I am only 23 and healthy otherwise.
Thanks! Autumn