I need answers

Late august I tested positive 2 weeks later on the day of my first ultrasound I started bleeding and there was no sac, no sign of a pregnancy.

I had miscarried

out of curiosity I took a pregnancy test 4 days ago and it came out positive I was super confused because I had just miscarried. I was excited and hoping this one would stick but unfortunately this morning I woke up bleeding and decided to take a test and it came out negative

What is going on with my body? Is there an explanation to this?