What Causes Heavy Bleeding In The Middle Of Microgynon Pack?

Hi, I'm 24 and have been on Microgynon for about 7 years now. This month half way through my pill pack I have started randomly heavily bleeding without any cramps or pain, I have been caught off guard a few times. The bleed is like a normal period, very heavy and lasts about a day at a time. I'm a bit concerned and don't really know what is going on.