brown spotting week before period scheduled

I have had brown spotting the week before my schedule period for a couple cycles now. I have been on low-ogestral for over 8 months. I noticed my last cycle's period was heavy like I was not on the pill and then I had very tender breasts after my period. Now I am at the week before my period and I have brown spotting again. What could be wrong? I am 36 years old.

Thank you.