I had an IUI when I ouvlated but my RE couldn't see a follicle?

I got an "almost positive" ovulation tuesday night (it was too close to call) and then Wednesday I got a full on positive on two different tests. So I scheduled the IUI at 1 that day. Well so they did the IUI and after wards she checked my ovaries and said she couldn't find anything. She said since I got a positive the night before that I might have just ovulated right before I came in for my appointment. She did bloodwork on me just to be sure I wasn't having a false positive. She called me back and said I did have my LH surge and that I was ovulating after they got the results. Well shouldn't she have seen like a ruptured follicle or something then, even if I did ovulate? She saw nothing and that really concerns me. My follicle was somewhat small 15mm only 3 days before this. It was growing on average of 1.25mm a day so it might have been around 18 mm on the day I ovulated. Is that too small? Did I miss the window for the IUI?