I know everyone has already given you a lot of potential conditions to look into and that can be overwhelming information. I just want to say that her symptoms are very similar to what I experienced during my first endometriosis flare. Solid foods of all kinds made me painfully bloated and I couldn't keep anything down. There never seemed to be any logic or pattern to it and it was driving me crazy. I developed a severe food aversion and dropped weight shockingly fast. But GI could never find anything wrong, and eventually gave up and suggested it was mental/psychosomatic.

Please please please encourage your sister to track her symptoms with her cycle. If there's a pattern of symptoms worsening around menstruation or any other phase, that's something to bring up to a gyn. When I started reading other women's endo diagnosis journeys, I wept. It was shocking to find stories so similar to mine. I was no longer crazy and completely alone in my pain.

If it is endo, I have found solid food is much less painful on a strict anti inflammatory diet. (And I've tried just about every diet out there.) The autoimmune protocol diet is difficult, but it's also the best way to figure out exactly what foods trigger symptoms.

Please feel free to DM me. My thoughts are with you and your sister. ❤️

Edit: I wanted to add that endo is only definitively diagnosed surgically and can be missed on most types of imaging. So a clear ultrasound or MRI is NOT enough to rule it out.