Question about pills

Hello! I’ve been on kurvelo combined pills since I was 14 ( I’m abt to be 21) and a few months ago I started noticing I was getting my period a day later than usual (usually it starts on the third day of the placebo pill on Monday) but it’s been starting on the 4th day on Tuesday.. and it’s started getting lighter and shorter. And today it’s just all brown blood. For context I take the pill everyday at the exact same time ( sometimes an hour before or an hour later but that’s rare ). I’m just wondering if that’s my body just getting used to the hormones or possibly a problem? I’m not exactly sure, I have a doctors appointment in January so I’ll make sure to bring this up!