I am asked this question several times a day. If you see someone with their arm in a sling, you assume they have injured their arm. If it's someone who knows that I spent the past year fighting breast cancer, I answer, "Nothing. My plastic surgeon wants me to avoid using that side for a while."
Sometimes, though, it's someone I barely know, or only know via the phone at work. If I meet a customer face-to-face for the first time and they ask, "What did you do to your arm?" I try to answer halfway and let it go. If you tell someone "I had breast cancer and just had step 2 of the reconstruction process and radiated tissue can be tricky when it comes to healing so I need to limit the use of the pectoral muscles on the left side for a few weeks to give my body an advantage and...." you see where this goes into the realm of Do They Really Need All That Information? pretty quickly.
It makes people feel like they should say, "Oh no! How are you doing? What stage? Are you OK now?" or even worse, it makes them completely uncomfortable and they look down and regret asking why my arm is in a sling. I hate making people feel uncomfortable, and cancer has an amazing ability to do that.
Today a coworker answered the question for me by saying, "Her arm is fine, but she got a new boob." The guy who asked looked puzzled and asked, "Is the sling holding it on?" I nearly died laughing. I enjoy working around people who have the same sense of humor that I have. This is someone who we know quite well and she knew she could get away with it. Hopefully, I'll get rid of the sling on Wednesday and people can stop asking.
On a side note, the question is sometimes, "Wow! You really changed your hair!" heh heh. No. No I did not.