Sounds like it might be galbladder or possibly appendix--do you still have appendix??
My son cant eat grease after taking out appendix. Would be the same if its not working correctly. My sister in law cant eat greasy at all after taking out gallbladder, would be the same if gallbladder is not working correctly..
Ibs I have that too, but it usually affects me from spicey food, not greasy--but all pple are different. And I usually get constipation, but some pple get diarehea, or back and forth.
You could ask for u/s of your gallbladder, kidneys, appendix, liver, spleen, and if it is any of those organs they could see on the ultrasound.
Good luck let us know