What Is The Cause Of Intermenstrual Bleeding?

hi there, am a bit worried about my periods, always been a bit irregular but settled down after my first child. in between my second and third child periods lasted for three days, stopped very suddenly, then bled again around a wek later, for 4-5 days, very sticky dark loss, but quite heavy. i had ovarian cysts with second and third pregnancy and had an ovary and tube removed at section with my third child child, as cyst was v large and not just a simple fluid filled cyst. periods now back to bleeding for 3 days with horrible dark loss around a week later, is this ok? feel like i'm having 2 periods a month! thanks, fiona campbell