How accurate is ultrasound in determining if ovulation has occurred?

I usually ovulate between cycle days 26-28 of my cycle; very rarely is it any later than that. I started Clomid for the first time this cycle on days 5-9. On days 8 and 9 I had ovulation type cramping, and on days 9 and 10 I had stretchy CM. Since then, things have dried up and I haven't really felt much in the way of uterine cramps. I went to my RE for a mid-cycle ultrasound yesterday (CD 13) and he said he didn't see anything - that it didn't look like I ovulated, but that I didn't have any follicles in there. He seemed to think I wasn't going to O at all, or else I would very late. I asked him if what he saw coincided with a 26-28 day ovulation, and he said it was possible but sounded very doubtful. So now I'm wondering - how accurate is the ultrasound in determining ovulation? Should I get a progesterone test?