I posted in the pregnancy section about almost the exact same thing just 2 days ago! I have never had a c-section but I am 38 weeks pregnant with my second and for the last few days I have had a very sore spot on my bump, and the only way I can describe the pain is that I feel like I have hit myself and I am bruised. But there is no sign of a bruise or anything on the skin.
I wonder what it could be, I think you should go and get it checked out and then let me know as well! When I called the midwife she said it sounded muscular and as long as baby was moving around fine and I wasn't too uncomfortable then I shouldn't worry...like you it is more annoying than anything, although it is a bit sore and painful as well.