What Causes Brown Vaginal Discharge While On Microgynon?

hi, been on microgynon 30 pill for about 2 years and have always had fairly heavy regular period. had bad month last month and forgot take about 3 and at different times of day often. thought was coming on on sunday but only had brown discharge and very little for about 3 days. has now completely cleared and have clear discharge. never had anything like this on pill before and was wondering what was causing it.