Light spotting

Hey ladies I’m 5 months postpartum and I pump and breastfeed. I have PCOS and I just recently started back taking metformin to regulate my hormones. I started medicine 6/16 but on 6/24 I had very very light pink spotting that I saw only when I wiped followed by very mild cramping. I thought it was my period about to start but I haven’t had the spotting or cramping since that day. I thought it may have been implantation but I tested yesterday and I was negative. Has anyone else experienced this? What was it?

Not my picture but it looked like this (a little lighter and with a bit of mucus)