My girlfriend is suddenly having severe mid and lower back pain. more so on the left side. what could it be?

Multiple reasons: Pain could be from an ovarian cyst. Where you recently sexually active?, pain could be from painful intercourse as a result of friction or trauma. Was she lifting anything or doing anything that would create pressure on the back? Pain could be from a strained muscle to a herniated disc. Asked her if she's ever had a ruptured ovarian cyst, especially with her history of iron deficiency anemia.