Nipple pain and sack in nipple

I'm 17, and I've had this pain in my right nipple for awhile now. I've already had pains in my nipples before, the whole puberty thing, but this seems a little different, though I hope I'm wrong. The nipple is almost always hard, and it's very sensitive like they both were during puberty. The difference, and what worries me, is that when I feel around/into it there is a really solid sack underneath that the other (that's not hurting) doesn't have. The sack is what's painful. It's very... almost inflamed? The sack is about the sick of my nipple itself but seems completely independent as I can feel the edges and how large it is. Any idea of what this is? Is this still just a puberty thing?

[Caryopteris]

It sounds like a benign condition called gynecomastia.  Men can get breast cancer, so it needs to be checked, but breast cancer is only occasionally cancerous.  

Gynecomastia is often caused by medications.  It is suspected even marijuana causes it.  Check wikipedia for more information about gynecomastia causes.