What Causes Black Color Menstrual Bleeding?

hello im a 20 year old female i have a 18mo son an had one abortion on march of this year. the last two month of my menstral cycle hasnt been normal last month it only lasted 2 days and then the rest of the 3 days was black dark blood. now this month it lasted 6 days and now it has began to start again but with the black blood. could this be any possible sign of any infections?