What Causes Lower Back And Pelvic Pain?

I m having severe lower back and pelvic pain when I sit for long periods on not cushion flat surface. I m a teacher and sometimes tutor but lately can t even sit in a student chair without feeling like I can t get up bc the pain is strong. It happened today and could barely walk to my car. I m laying in bed now. Took two Motrin and out Bengay.