Spotting in between periods

I am 10 months post partum, breastfeeding but my period came back 3 months ago, so I’m on my third cycle. I am half way through my cycle abs I’ve had spotting, bright red blood in my cm. It’s not enough to stain my clothes just when. I wipe and the tiniest amounts. It’s been like this for 4 days. About a month ago I had a cervical smear test and everything came back negative and they said everything looked normal also. This has never ever happened before- what could be causing this? I’m not pregnant either.