Back pain. Most common cause is muscle strain (pulled muscles). There are several other spine related causes such as disc herniation and arthritis. Non spine causes include kidney infection/stones, gallbladder stones, pancreas inflammation, ulcers, etc. Urinary tract infections, pregnancy, ectopic pregnancy, pelvic infections are also causes to list.
There are many. For women, not only do you have to consider the bony structures, muscles, nerves (same as for men) but also organs that can refer pain to the back including the uterus, ovaries, cervix.