I have had a weird lump growing in my right breast for the last 3 days. It is rather deep in my breast, if you know what I mean, not right on the surface, but closer to my rib cage. It is now roughly the size and shape of a ping pong ball cut in half. It is very tender and sore. I have had 2 plugged ducts in the past, but I have always been able to massage them away very quickly and they never got this big. Those were also right on the surface, though. You can't see anything (no redness, obvious swelling, etc), but even my husband could easily feel the lump I'm talking about.
A little further info, which may or may not be relevant--DS has been bf'ed for 15 months, never had any problems. I have been cutting back on his feeds slowly over the last month or so because I start back to work again tomorrow and don't want to have to pump since he won't take a bottle at this point anyways. I do not feel engorged, nor does feeding him off that breast seem to give me any relief.
I'm kind of getting freaked out here, and worrying about cancer (my grandmother died of breast cancer the month DS was conceived). I know that in all reality, that's not the case, since it sprang up too quickly, but it's the only thing I can think of, since my symptoms don't really match with what I know of mastitis or my former experiences with plugged ducts.
Please give me ideas about what else this could be and what I can do to get some relief. And some hugs and "don't freak out vibes" would be great too!

Thanks.