Spotting, breakthrough bleeding or what? on bc?

i have been on ortho tri cyclen lo for over 2 years. i'm due to start my period on thursday. however, for the first time ever, 2 days ago, on friday, i noticed some brown discharge - possibly old blood - in my panties. i came home and used the bathroom and when i wiped i had light red mucousy discharge. i thought that i had started my period early so i put on a pad. however, i have not had any more bleeding. does anyone know? i have taken my birth control perfectly.