What Causes Black Mucus Discharge While On Second Week Of Contraceptives?

Hi there, I AM going to the OB this week but in the meantime I'd like some theories. I am 37 years old, have 2 children, am taking orthotrycycline (for several years now). Typically on the last day of my active pills I may have some blood tinged spotting, but this month in my second week of pill cycle I began to have a black, odorless, mucus like discharge. Enough of it that I've had to wear a tampon, not soaking (it wouldn't though because of the thick texture) but changing tampons twice a day. It's been like this for a week now. I'm worried about a cervical cancer or something. Any thoughts I can research while waiting for my appt.?
Jennifer