HELP! Mysterious back pain

I've had terrible lower back pain with intermittent cramps since last Thursday. I have the Mirena IUD so I'm not sure if that could be the cause? I did a string check and they haven't moved. I took a pregnancy test for peace of mind and it came back negative. So did my urinalysis, no kidney infection or UTI. (TMI) I've been completely regular so I highly doubt it's a gastrointestinal issue. The doctor gave me a steroid shot and that only made things worse! What could this be? I'm in tears it's so bad, calling the doctor and taking off work in the morning. Help!