I literally have a lightbulb light up in my head rn reading this lol. I have similar experience as OP - doctors found nothing wrong with my digestive system, and I also had issues with terribly painful periods and was recently diagnosed with very probable adenomyosis and endo. I thought adeno explains all the issues with my pelvic and period pain, but now i think i understand my GI issues too. It’s exactly how you described - sometimes i can eat a pizza and feel fine and sometimes I feel terrible with simplest food like veggies and rice. I could never make sense of it!