When Kayleigh and I were messing around quite some time ago, I ended up breaking a rib right near my sternum. It started bothering me a lot a week after I did it, possibly because I was carrying a bag with 3 textbooks in it around uni. I went to the uni doctor where it was confirmed what I had done. It hurt to breathe in especially, and to pick up my bag.
The only treatment was to take Panadol. I did a couple of times but nothing happened so I stopped. It got better after about 5 weeks.