My boobs have been weird since I finished bf-ing DD (just over a year ago), they've been getting rather sore and I convinced myself I'd found a lump in my armpit so off to the docs I went in a panic
I explained symptoms, stressed I had a lump under one arm, which I could feel if I lay slightly on one side with arm up etc etc. I (prompted) stripped off and she (thankfully she!) checked for boob lumps etc, didn't find anything, I demonstrated the armpit lump, and she (impressively straightfaced) said to me:
"Yes, you'll find you have that under both arms, it is your latissimus dorsi muscle." 
How did I not notice that?