What Causes Severe Pain In Sacro-iliac Joint?

Hi I am a medical doctor, (Internal medicine, primary care), for over 30 years. age; 63, (HTN, mild renal insufiency, Crt 1.37/GFR 55;Meds: Bystolic, (beta blocker) 10 mg/d, Norvasc 10mg/d, Lasix 40mg/d Question: I have FLAT FEET (literally). for the last 3 weeks I have severe pain in my sacro-iliac joint, more than in my hips. Treated with stopped working 2 tabs tid , the last week started Naprosyn 500 my bid. For the 1st time in my life started Tylenol #3,. bid. After 3 since this started I had aut 5 minutes free of pain today!