For my IUIs my RE always had me start p4 support 2-3 days following the IUI. He also told me to BD that night for good measure. Do you temp? That's a good way to know for sure whether or not you O'd.
So, yes, 2-days post IUI for p4 support is standard. And I think BDing tonight is a good idea
Sorry to hear your IVF was canceled, but it's awesome that you've got three good follies in the running.
Hugs and GL!