What kind of implants do you have?
I have teardrop/textured implants. Got them in 1999. In 2004 after weaning DS1, I discovered a tiny hard lump (like a bb) and went in for an ultrasound and then a biopsy. Turns out it was a "silicone granulomata" from the implant itself. Basically, part of the silicone "casing" of the textured implant shell had sloughed off and formed a lump in my breast. Gross.
So maybe it's something like that? Talk to your OB about it.
I can't wait to get my stupid, shitty implants replaced in March. I wish I had never gotten them in the first place.
(Or at least waited until I had moved to southern California and done it in the plastic surgery capital of the world!)