4 days late - negative tests! Is it due to breastfeeding still?

I am 4 days late for period and am getting bfns on stick tests and on the cb digital I took yesterday. This would have been my third period post partum - my others started 1st Dec and 1st Jan. I still breastfeed once a day. My cycle was very regular pre baby but I always ovulated quite late - I’ve not confirmed ovulation post partum.

What do you reckon? Anyone else experienced this? Is is just irregular cycles as I’m breastfeeding? Surely if I was pregnant I’d get a line by now?