Bleeding on BC?

VMR

So I had my son on Aug 20. Had my PP checkup on sept 19 (4weeks after having my son) and started BC on sept 22nd. My husband and I have been having sex since 2 weeks after my son but on sept 30 after we had sex I started to bleed. It was red but old looking blood, and I still have it. I’d say it’s heavy spotting.

Any ideas on what it could be? I’m barely a week into my pack so I have no idea, it’s not my period.