I gave birth to a baby girl 3 months ago and I had 1 normal period and now I'm 18 days late,what's going on?

Sadly just 3 months ago I gave birth to my beautiful baby girl at 7 months but she immediately died of Caudal Regression Syndrome :( and it devastated my fiance and I.After I gave birth to her I bled for 2 weeks and then 21 days later i got my normal period which lasted 5-6 days.My fiance and I has unprotected a lot and I was supposed to get my period 18 days ago from today.I took a home test it was negative and i went to my doc and i was given a test there and it was also negative so what is going on with me!?