Hi. Sorry she is in so much pain. I am not a doctor.
pain under right rib cage, sometimes radiates to side and back- worse during period. No popping. do get a Dull nagging pain and sometimes pain which makes it painful to move in certain positions. Have to get comfortable position. Sometimes it can be sharp and stabbing, intermittent with dull and aching. It can last for 2 weeks+.
I have put the following below for possible pain management pending diagnosis together with a small list of things that might be causing the problem. A diagnosis is required but if you can rule in/out foods that affect the pain, then it may help you and your doctor come to the right diagnosis.
In a nutshell: Drink Cranberry Juice. Change diet no carbs more protein, Drink a little water too to keep you hydrated. Cut out sugar pronto. Cut out gluten products (flour,pasta,bread & potatos,biscuits and crisps, cola drinks etc) to see if this reduces pain. (a diet of chicken and rice and veg/salad for time being).Of course this also means no beer!
The Drs first always try to rule out medical emergency problems, like gallstones etc. Your sister should go back to her doctor regardless. upper and lower abdomen ultrasound. Clear. Blood test. clear. Urine test. Clear. Keep checking with your doctor as regards to symptoms but perhaps try a change of diet to help manage the pain.
pain experienced mainly a week before, during and a week after period. lucky to have 2 weeks sometimes where pain not occurring. Is this the same for your sister? It is perhaps therefore hormonal/enzyme related and made worse by foods (like sugar, sugary drinks? Bread/pasta/potato?).
If you find that sugar and/or flour products cause pain to increase then please inform your doctor.
Celiacs (gluten intollerance that can cause damage to the intestine)-need to avoid gluten for life!
Pancreas problem-hormones and sugar intake can be related.
Candidiasis (over growth of candida albicans ask Doctor to check for this.
Antibiotics and antacids are not recommended and can increase the candida.
Endometritis(spelling?) is another possible cause which may need to be ruled out?
Good luck with this and of course keep smiling!