Why do I have a period?

I started the non-combined pill (called Cerazette in the UK, not sure if it has a different name elsewhere) around the start of September. My doctor said women don't usually get periods while on this pill and I have not had one since I started it, however I go to the toilet today and this has happened! Its not a full period and It's a lot more brown than in the photo, a bit like BBQ sauce colour 😂 has this happened to anyone else or anyone have suggestions? Thanks