if you are giving complete breast feeding the menstruation
will return late, but as the periods started already there is less chance for contraceptive
benefit from breast feeding,
but as your pregnancy
test showed negative result there is less possibility for pregnancy,
tablets sometimes can cause brownish discharge sometimes as side effect, as they cannot stop already established periods you can continue the tablets, if normal flow of periods develop you can stop them, if not after completion of tablets in around 5 days you will get withdrawal bleeding.