That makes me sad =( He is very active, loves to play, run and laugh!! this won't help him with sports and activities. I hope if it is that, that it will go away soon!!!
They probably cut a nerve during his surgery and that's what he is feeling now. I had the the same thing happening to me, but in my case the pain is on the left side and it wan't heart surgery. How long is that going to stay with him? I don't know, my surgery was more than 15 years ago and I still have it. I am just trying to avoid all motion that will trigger the pain. Doubtful you can teach that to a 5 year old.
I have taken him to the pediatric, we took him ultrasound as well and all is clear. The pediatric isn't sure what it could be, she said maybe constipation? But we are going to start him on a pediatric laxative.
There is no way of knowing what it could be. Is it heart related; the cvhances oif that are basically nil; nothing you have written on here directs me to believe that. Take him to the pediatrician.