Chemical pregnancy

Does anyone really know why a chemical pregnancy happens? I had one 2 cycles ago...

I had positive pregnancy test & was confirmed by a doctor.. I was only 4 weeks my hcg levels were very low and I ended up getting my period.

What a strange thing to experience it's like omg I'm pregnant!!!! Then u find out it's not a viable pregnancy. Like oh ok... Guess I'm not pregnant.

Chemical pregnancies are just a strange thing...

I was almost pregnant? Lol

better luck next time I guess.