What's going on?!! 😭😭😭

I had my daughter around 4 
months ago. I stopped bleeding 2
weeks after delivery, and when
she was 2 months old started
bleeding again. The bleeding lasted
5 days, so I assumed it was a
period. Exactly 28 days later I 
started bleeding again, for 6 days. 
So I know it was a period. 
I'm now about a week late for
 this (third) period. I have been
having unprotected intercourse
since 3 weeks postpartum. 
We don't do the pull out method
either. I had a positive pregnancy
test 4 days ago, but since then
my tests have been negative.
What's going on?