Brown discharge

Okay so on & off I’ve had like brown discharge people say it’s old blood but old blood from what? I’m 15 weeks PP haven’t gotten my period (due to breastfeeding) I don’t have any symptoms of a bacterial infection or yeast infection or anything no itching, burning NOTHING. I clean myself daily I’ve been using summers eve for years & OBVIOUSLY I don’t put it in my actual vagina EVER it’s not heavy it’s just a bit of discharge you know? Like a regular amount. I know there aren’t may drs on here but I’m just wondering if anyone has any ideas?