Definitely talk to the pediatrician. She may have a "nervous tummy" or Irritable Bowel Syndrome. I'm not a physician, but I was extremely thin as a child. I had my share of tummy troubles. My parents just blamed it on me playing too hard before or after dinner.
That said, I have a very tiny mother. She is very short and petite. I got my height from other family members. However, I'm 5'7", 50 years old, and after having two children, I weigh 115. My daughter weighed 5 pounds full term and she is the cutest little pixie at 27 years old after having a child. 5 feet tall.
The one difference with my daughter is that her pediatrician was never worried about her weight because she was so incredibly healthy and thriving like crazy.
I think your best bet is to ask for a referral to a nutritionist to make sure your daughter is getting her digestive needs met and can help you with recipes or her pickiness issues. She may be picky if certain things upset her stomach. It's worth looking into further.
Unless the pediatrician feels she's "failing to thrive", I wouldn't worry too much and just seek the advice of the professionals.
Just my opinion.
Best wishes.