While breastfeeding properly in its way periods usually don't happen and is known as lactational amenorrhea. It's also a good habit to have a kind of birth control pill while breastfeeding which prevents accidental pregnancy. It seems that something is wrong and hence your periods are irregular in the form of brownish discharge...
Next Steps
You should immediately consult your doctor and ask for an advice after providing more information in your history...
Health Tips
Don't neglect and know the right way about what to expect and what not during the postpartum period. All the best.