Common causes shoulder blade is pain due to muscle strain ,rotator cuff tear ,bone fracture ,connective tissue ie tendon rubbing on shoulder blade you can do rest ,apply ice ,or heat ,medication ,massage, pain in sacroiliac joint occurs when ligament become painfully ,or too loose or too tight it result due to injury (accident ,pregnant ,hip /spine surgery ,laminectomy ,lumbar fusion )pelvic brace /belt can be helpful in case joint r inflamed and pain ,stretching exercise ,losing weight can help.