Follicles and eggs help!

Hi everyone,

I’ve been taking puregon (injectables) for a week now. I went in today for blood work and ultrasound. When I was there the nurse told me I had 3 great follicles and my lining and etc were great and that we should be doing

IUI

this weekend. I just got a call that I need to come back in because I’m ovulating sooner but they don’t see the egg to come back for another ultrasound. What does this mean!! Thank you