Yeah, been there and had this - albeit over 20 years ago LOL A kiddy you are carrying on your hip, kicking you in the muscles around your rib cage and under your rib cage, can cause pains and dull aches - cause and effect. Add to that the fact that it is causing you to over use such muscles on one side and walk differently with a weight on one side and voila. I would suggest doing what I did, swap sides, make sure you equally carry on both.....also the kicking on the chest....how old is he ? If he is half your weight and you are tiny, and its causing pain, I do think you need to reconsider how much carrying you are actually doing.
Well of course it will, whats the chances of it coming up with - small child being carried and kicking in the ribs
Thats like telling a doctor about your pain (but a rubbish doctor) and neglecting to tell them the exact events that are causing the pain ! Oh and it won't be doing your spine much good either.