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Three possibilities come to my mind; chronic involvement of appendix (many a times ultrasound misses this as the appendix in such cases is hidden behind loops of intestine and typical symptoms of appendicitis are absent), involvement of kidney and inflammation of your intestine. Crohn’s disease, diverticulitis, infection in kidney, kidney stones, constipation, ovary involvement in women, ectopic pregnancy in women, mesentry (a thin tissue that carries the blood, nerves and lymphatics for the intestine) involvement, and an abscess in rectus abdominis muscle or psoas muscle can all cause lower right abdomen pain. Go through this and see if any of these were missed. If you had surgeries in the past then the pain can be due to adhesions.
It is difficult to comment beyond this at this stage. Please let me know if there is any thing else and do keep me posted. Take care