How To Treat Pain In Abdomen And Lower Back Post Miscarriage?

I had a miscarriage 3 weeks ago. My abdomen, sides and left kidney area in pain and lower back pain. Ultra sounds show I m all clear and urine tests show no kidney infections. Still I havnt been doing any heavy lifting to cause any muscle pains or strains, but the dr can t tell me why I m in pain,, he said gastrointestinal and I think he has no clue why I m in pain and so sore still.