HELP! Period or old blood?

I have been on the mini pill for a year now. The first 6 months I had fairly regular periods - they were much lighter than I was used to, but they were bright red, 3-5 days, etc. The last six months have been different. I haven't had a true "period" in the last several months. I will almost constantly have dark (almost black) discharge. Some days are heavier than others. Is this due to the pill? Should I be concerned?

*I have a gyno appointment but it's in a month.