D.W.
I agree with Amber's advice.
However, I tend to also agree with your pediatrician that she may have a much different personality. Our daughter is 21 months, and she is the polar opposite of her brother. She cries at the mention of the word "NO".
Is she verbal? Both of our kids were a little later with their ability to talk, and we had to ask them to "Show Us" what they wanted if we couldn't understand what they were saying.
It doesn't hurt to get another opinion, though, if your gut tells you otherwise. I found a swollen lymph node that the first doctor "wasn't excited about". My gut told me otherwise, and I was diagnosed with cancer. Not to say your daughter has a grave disease, but our Mom instincts are strong. Even if it's just her personality differences, it may give you peace of mind.