Google can't help me.

I'm gonna turn to glow. 
Haha so I had an ultrasound and my uterine lining was at 9mm. 
5 days later I had another ultrasound and it was at 12.48mm 
4 days after that ultrasound I had another and it was down to 8.75mm. 
What causes it to go down like that? 
Ovulation didn't happen last cycle. Clomid didn't work for me. So I don't know what would cause that and Google isn't telling me anything.