I wouldn't be too concerned hun, and I wouldn't introduce solids yet if she has been having long periods with no bowel movements.
I was advised to start solids early because DS was a slow weight gainer, but he is fine, he is just small compared to most kids. You need to consider the genetics she has and whether she is going to be 'big'... what were your other children like at this stage?

And I agree with em... unless they are weighed naked and on the same scales, then I don't take too much notice as there are too many variables