Back Pain - What does lower back pain cause in early pregnancy?

What does lower back pain cause in early pregnancy?

I understand the reason for back pain from 4 to 9 months of pregnancy but exactly what causes lower back pain in 4th or 5th week of early pregnancy

I don't know, but I have it too! You're not alone :)
I want to say it is from the pulling, and stretching of the uterus. It even pulls some of the muscles in our backs. I think.