usually after delivery if you are giving complete breast feeding the periods will delay and also it will take some time for regularization of the periods, in case of non breast feeding women the contraceptive
benefits of the breast feeding are doubtful,
you didn't mention regarding your menstrual cycle, so your bleeding could be part of menstrual bleeding or post ovulatory bleeding or in case of unprotected intercourse
around the time of ovulation there is possibility for implantation
so your better check for pregnancy
test once, if comes negative possibly the spotting will subside or turn into menstrual flow.