Can lining shrink 2mm?

As

On CD 14 I had 22mm follicle and 8.5mm lining.

CD 15 triggered but potentially had already ovulated.

CD 16 ultrasound before iui confirmed ovulation. Iui cancelled.

When I got home the patient portal notes say my lining was measured at 6.5mm today.

Would it shrink 2mm in 2 days? I start endometrin suppositories tonight which I do after every iui but I'm worried that implantation won't be possible at 6.5mm.