A few weeks after I gave birth, I can't remember how many exactly, I found a small lump in my upper breast area. This was the first big health scare for me (excluding the labour and c section), and it sent me into a spin. Now, I wasn't able to breast feed, abut the lump was there none the less. I was referred for an ultrasound and a meeting with the breast doc. The doc said it was fine and absolutely nothing to worry about. Then by the time I had the u/s, it had all but disappeared, so looking back I'm guessing it was either a cyst, a blocked milk duct or a lymph node. Either way, I'm guessing since 3 of us have had this, it must be pretty normal post partum.
I'm totally with you on the lymphoma freak out. I'm still dealing with it because of enlarged nodes in my neck.