What Causes Lower Back Pain During Menstrual Cycle?

last year i started to get some lower back pain during my periods usually would subside during period one month it never went away.had lower back pain for 2 months then during that period it went away-every now and then still get some lower back pain with onset of period,3 weeks ago had my period and no lower back pain.period was normal except still wiping brown blood sometimes pink but mostly brown.now after 3 weeks the lower pain in my back is back but been moving down into what seems like right side of tailbone -hurts when i sneeze or cough.im 46 years old and have had 3 children.what could be going on with the pain and brown blood especially after 3 weeks