It could be gyno... Hard to say especially since you say you haven't been working out. I had a very mild case of gyno, wasn't too much bigger then yours and had the surgery done about a month and a half ago. Mine was from puberty (not sure of your age). I would definitely recommend working out for a few months, get in shape, and then see if it is still there though.
If it is still there, I would recommend getting the surgery as soon you can. I finally got the surgery done and I'm 27... Wish I would've had the courage to have done it 10 years ago. It sucks thinking people are staring at you when you take your shirt off, even though most people in reality don't even notice anything.