What Causes Vaginal Bleeding In Between Periods While On Birth Control?

Hi, I am 18 years old and have been on birth control since I was 15 (initially to regulate my period). Yesterday was the 9th day of my cycle and I experienced spotting for the first time ever- all day when I went to the bathroom there was a little blood when I wiped or on my underwear. This continued into today and I am worried because I have not bled in between periods ever since I've been on birth control. I have not recently switched pills. I was wondering what this may be and if I should worry. I have a doctor's appointment for Tuesday. Thank you.