Can an ultrasound be wrong?

So either I am pretty late ovulating, or I am 3 dpo (days past ovulation). I have no idea which one and I'm not sure what to think. Here's the deal: I had my mid-cycle ultrasound on day 18 (last month I ovulated on day 21 or 22). I had gotten what I thought was probably a positive ovulation test the day before- day 17. It was definitely as dark as the control line, though not darker like last month when I had a positive ovulation test. So I was unsure, and I told the residents when they did my ultrasound. They looked, and said they didn't think I had ovulated, that my lining was still thinner than it should be (7.5, when ideally it's 8-12, but at least I was above 6, what is considered the minimum), and that they saw one huge cyst (40mm) and a bunch of small ones, like around 11-14 mm (I can't remember if they said 11 or 14. forgive me, but it was 7:30 am and I am not human yet at that hour. I was also nervous and upset, as always at the doctor's when I have a huge wand up my wazoo).

Anyway, they said it looked to them like I had not ovulated, but that I was on track to ovulate on Monday, day 22, just like last month.

I accepted this, and carried on, and have continued to do ovulation tests. Today is day 21, Sunday, and I have not really gotten a positive test yes (today's was darker than normal, but not as dark as Wednesday's). However, this is the weird part: I got that positive test last Wednesday, and my temperature spiked on Friday morning. It's been high each day since.

So it does seem that I might have ovulated. I don't know why it wouldn't appear that way on my ultrasound, except that they don't seem very reliable to me. More an art than a science. Last month they told me I had already ovulated, when in fact I wouldn't for another week. So, I kinda think those residents might be, uh, not that good at ultrasounds. Or it's just hard to read. Or they did say that big cyst was making things really hard to see.

So, I'm trying not to obsess, and we're just continuing to try to make a baby every other day, and I'll keep peeing in a cup and temping. I'm hoping that I really am 3dpo and not just not ovulating. I think I will email my doctor tomorrow and ask, "Can an ultrasound be wrong???"

Has anyone else ever gotten mixed messages like this? Or had a similar reason to doubt the ultrasounds???

Thanks for reading, whoever you are. Good night.