You probably have bleeding gums. It is highly likely that you could have a gum disorder or sensitive gums that react to brushing or flossing by bleeding. The bleeding is probably so slightit can only be noticed after it has had time to build up in your mouth like when you are sleeping.
You should go see your family doctor if this continues...you might have a ulcer that is starting to bleed so you are tasting the blood when you wake up in the morning. Please don't put this off .There some good meds. Available over the counter and prescription ones too. Djoyful
Probably due to not brushing your teeth properly and causing bacteria to infest your gums causing irritation and bleeding.