There could be multiple reasons for back pain, around the time of your period:
1. Not drinking enough water - this can affect the whole body system, and is especially important around your period.
2. Circulatory problems can cause aches and pains - key symptoms are cold hands and feet.
3. Posture could be a factor - we tend to hold ourselves differently around our period - slouching etc.
4. Old injuries can flare up around the time of the month, as water retention can cause added inflammation.