See PJ's answer to this question posted yesterday. I'll also throw in my recommendation to see a doctor. If this lump gets larger and then smaller with your period, then it would probably be a cyst. In fact, it probably is a cyst. It almost certainly isn't breast cancer, so try not to worry. However, a lump that has been growing for six months needs to be checked out by a doctor for any person at any age no matter where it is.