I'm well over a year past bypass and have two bumps exactly as you describe. I think it is where the sternum is wired shut, but could be wrong. That said, I'm sure it is harmless.
I had another tender, soft bump and the doctor removed a tiny bit of thread that was supposed to dissolve but didn't, it also could be that.
Your chest will be a bit sore for quite a while, it's usually quite to be expected. Remember, four months ago your chest was sawed open, then the ribs stretched open, that is a ripe moment to have a lingering soreness.
Hope your heart recovering.
Hi, the bump is very common. Took a year for mine to go away. You will feel tenderness and all kinds of strange sensations as the nerves and wounds heal. It took exactly a year before I felt normal again.