What Causes Brownish Gooey Discharge After Taking Contraceptive Pill?

I am on birth control but last month missed 3 or 4 of my pills, had my period, then had sex without a condom. I didn't miss a day of pills after that period. I have now had a brown/pink gooey discharge for about 4 days and my period is not supposed to start for another week. What can be the cause of this?