there can be many reasons. do not try and diagnose your elderly stubborn mother. try and convince her to be properly examined and diagnosed. there are many other things in this area: kidney ureter liver gallbladder intestines etc. it would be wrong to assume a rib separation.
my mother is 91 and has started to have severe pain in her lower ribs on her right side. she will not go to the doctor. she's using bengay patches and t. ramadol. she has not fallen or injured herself in any way. the only thing i can think of is rib separation when she turned in her sleep.