Welcome to the MedHelp forum!
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.
Lower right abdomen pain is commonly due to appendicitis. The pain, can at times come and go and not give the classic picture. Crohn’s disease, diverticulitis, infection in kidney, kidney stones, constipation, ovary involvement in women (cysts, hemorrhage, tumor, torsion—torsion is a medical emergency and a long standing pain is not due to this), ectopic pregnancy in women, mesentry (a thin tissue that carries the blood, nerves and lymphatics for the intestine) involvement low blood supply, enlarged lymphnodes, and an abscess in rectus abdominis muscle or psoas muscle (these two are unlikely is there is no localized patch of heat or lump) can all cause lower right abdomen pain. It can also be due to inguinal hernia. Other than that at times pelvic inflammatory disease in women causes a lower abdominal pain along with a pelvic pain.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP for primary examination followed by proper referral. Take care!
Too little information to give sound advice. See your family MD.