Brown discharge, back pain, and itching and burning of the vagina. what could this be?

You probably have infectious vaginitis, you should see your family doctor or preferably a GYN doctor. In the meantime you can do sitz bath with baking soda (1/4 cup of baking soda in 2-3 inches of water in a sitz bath), ten minutes each time, a couple times a day.

A bacterial infection. See your gynecologist for a diagnosis and treatment. Good Luck