Joint Pain. Of musculoskeletal pain?

Have been experiencing knee and foot joint pain for around 5 years. There has never been any visible joint swelling. I have already been evaluated extensively by a rheumatologist. HLA-B27, RA/CCP, ANA, ENA, tTG, ANCA, uric acid, CPK, bone scan and MRI were all normal. ESR and CRP have also remained normal. I have noticed a strong pattern with my bowel movements: whenever I become constipated, my knee and foot pain becomes worse. After passing stool, I get significant relief in the joint pain. Could constipation, IBS-C, pelvic-floor dysfunction, or another gastrointestinal problem contribute to this type of musculoskeletal pain? Please advise what further gastrointestinal evaluation or treatment you would recommend