Weird cycle?

I had a baby in September of 2019. I am exclusively breastfeeding. My period returned in February at 5 months pp, and it was heavier and longer than I would expect. According to my calculations, I should've had another period by now. I've taken 2 pregnancy tests in the last couple weeks, both 100% negative. The chances of me being pregnant are slim, but I'm just so confused. Is this normal?