What are the common causes of low back pain, menstrual cramps, and heavy discharge?

Depends: It probably relates to the shifting levels of hormones in your system throughout your cycle. If the cramping and back pain are mild, keep going and use over the counter medications if needed (assuming you have not been told not to take them for some medical reason). If the pain is severe or your discharge is very heavy, see your gynecologist or pcp to make sure there isn't a more serious problem.