Brown spotting/period

Was supposed to start my period yesterday but have only been having a very little pink blood but it’s mostly old blood. My normal periods are semi heavy. I’m almost 4 months postpartum from having my second baby. We had sex on my ovulation day and hubby came inside on accident. I’m also breastfeeding and lately my little hasn’t wanted to nurse and now I’m worried that it may be because I’m pregnant. Any advice ladies?