if there are any nurses here or I guess just anyone HELP! My son is 7 months old tonight he was…

if there are any nurses here or I guess just anyone HELP! My son is 7 months old tonight he was crying because he was ready for bed so i started to rub his belly and i noticed that his right rib sticks out alot more than the left. if you touch it there is definitely a difference. I nursed him and he fell asleep like usual. I don't know if it's normal or if I should be worried what should I do??