Strange bleeding while on birth control?

So for the past week I’ve been bleeding very lightly but consistently brown blood. I’m currently on Yasmin and haven’t missed any pills that might cause me to start spotting. Only other symptom is my breasts have been a bit tender the past two days. Haven’t had intercourse for almost two months so assuming it wouldn’t be implantation bleeding. Just wondering what this could be or if it’s something to worry about, thanks.