Fluid in uterine lining before IUI

I just had an ultrasound this morning, I've been doing Follistim injections of anywhere from 41-50IUI for 15 days. The follicles are finally ready to be triggered but the RE said today that I have a little fluid in my uterine lining. She asked if I'd been spotting, I haven't. She didn't get into what it could mean or how it got there. I'm just wondering if anyone else has ever had this happen. We're planning the IUI for Saturday unless the bloodwork shows high LH then it will be tomorrow. I'm just worried that it will affect the IUI and I'm not really sure how the fluid got there. Our clinic is not the best clinic about answering questions or telling us our chances of success. It's the only one our insurance will cover. I was just so happy that things were going so well, this is my 4th IUI and first with injectables. We were only getting a lining of 4 on trigger day before and this time it's 8.7 so I was so hopeful this cycle. But now I'm scared this fluid is going to inhibit implantation. If anyone has any experience with this it would be greatly appreciated!!