If you have been using micronor
for a long time, then its normal for your menstrual flow to decrease. This is because these pills cause thinning of the uterine lining with each cycle. IT is this lining of uterus
which is shed in menses. So with continuous use of micronor, endometrial lining (uterine lining) gets thinner and thinner making the periods lighter and lighter with eventually no periods.
If you have anemia
then this also causes bleeding to be lighter in menses.
Iron tablets can cause black stools. Its normal too. If you do not have any fresh bleeding or painful stools then you need not worry.