Tiny girl

So I had my baby girls  four-month appointment we found out she is only 1% for height and 0.5% for weight. She has always been smaller (even though she was born a week late) but this was an even smaller percentage then what she was at the two month appointment. I now have to go in again in a month to have her checked again. The doctor said she isn't worried yet because developmentally she is on track, but of course I am worried. She is exclusively breast-fed and seems happy and healthy but I can't help but worry about her growth and weight. Has anybody had any experience with this?  My other two children were never this small but they were both boys.