I too have stomach pain but not as severe as yours. I have autoimmune that has been diagnosed as RA however the variety of meds I’ve tried have either had unwanted side effects or helped minimally. I have neuropathy I have chronic kidney disease low iron. My joint swelling is minimal, but stomach is still a problem. I changed my diet went Keto then vegan , lost 50+ pounds but also lost muscle which at my old age has been a negative. So I have modified my diet. I find if I fast and let my stomach rest I start feeling good, but I am reluctant to go for extended fast due too my age and comorbidities I deal with. The last biologic I was on helped my autoimmune issues fairly well including stomach pain, for a year then my stomach developed the severe pain again. So I am not sure if next step. I am not very active due to all these health issues.
I hope you find a resolution and restore your health.