Do follicles shrink?

Risi

Weird question I know. I had a cd 12 follicle ultrasound and on my right side I had two 16-17mm and on my left I had a 19mm. I went back 24hrs later (cd 13) and the one on my left was 23mm but the nurse said she only saw one on the right side at 12mm. What happened to the two at 16-17mm? She made the ultrasound was so quick and I feel like she barely looked on my right side after seeing the huge one on the left.