I had mine measured last week as part of a pre-term labour study my hospital is doing (was also offered it in my last pregnancy). I was especially keen to have it done because I had a LEEP procedure some years ago, which removed a piece of cervix. I was fine both times, can't remember what the measurement was 2 years ago, but last week it was 34mm and tightly closed with no funnelling, and the sonographer was very pleased, low risk for pre-term labour on that count.
If the doc says there's nothing to worry about, you have to believe that. However if you're in pain, maybe you should go back? It's normal to have pains up there in pregnancy, though, I have had some that feel like I'm being stabbed with a hot skewer. They don't worry me now but they did last pregnancy, I was a wreck.