Confused. Please help.

Hi! I’ve been on the Yaz birth control pill for a year and take it regularly. I also always make my boyfriend use a condom when we have sex. The past two months about a 1.5 weeks before I start my period, I have been spotting lightly. Usually it happens in the evening. I have had no other symptoms of early pregnancy or anything. It is like a light pink or red color. Sometimes I barely notice it. What can this be? And what can I do about it to make it stop?