Help, please! Lactating but negative blood test

Help! I took two urine pregnancy tests on the day I was supposed to get my period, the next day I started to bleed. No cramping no clotting just light spotting so I went to the doctor to get a blood test & ultrasound, both were negative. And I continued to bleed the rest of the 4 days that I usually bleed on my period. A week later I am lactating & I am 6 months PP & I stopped breastfeeeding at 2 months PP & completely dried up at 3 months PP. I'm so confused! Can anyone relate or tell me what's going on?