Could someone break their own rib or hurt a lung if they punched themself in the stomach?

Break your rib?: It would be really difficutlt to do it yourself. It is just hard to develop enough force with the limited wind-up you can get to punch yourself. The ribs are pretty flexible. The case in which something like this might happen is when the individual has osteoporosis or a condition like cancer that makes the bones fragile. But even then it usually takes a punch from someone else.