What Does Random Bleeding While Being On BC Mean?

Hi. I got married in May last year. In October I decided that I would go on the pill. I know there are side effects with the pill but the only thing that has happened was that I noticed I have heavier discharge. Here in the past week I have randomly had apot bleeding that only lasted a day. I could understand if that happened in the begining, but I have been on the pill for four months already. What does the random bleeding mean? I already had my period a few weeks ago. This is new for me.